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Old 13-08-2022, 17:50   #1
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Prop for Electric Motor

Hi, I had my boat hauled out yesterday and will be having an electric motor installed this week. It used to have an old 4 cylinder diesel volvo in there that got about 70hp. (pretty big for the old 32' boat, but in the 60's, that's what they did)

Now, I'm converting it to electric and bought a kit:
https://www.thunderstruck-ev.com/18-...lboat-kit.html

and a gear reduction:
https://www.thunderstruck-ev.com/cus...reduction.html

The motor is the equivalent of about a 70hp, but unlike the diesel, it has all the torque it gets from 0 rpm and goes up to 6000rpm, so I understand that perhaps I need to change things up a bit.

The old diesel couldn't really handle a quick gear change. You kind of had to let it work it's way up, but once it got "a grip", it could push the boat 7-8k with no problem, but as the RPM increased, you could almost feel it slipping.

It's got a little 2blade prop on it and I'm thinking that, with the new motor, I probably should move up to a larger diameter and go with 3 blades.

Here's the question: How in the world do you decide??? There are tons of props on ebay (I've really gotta keep it under 1k, my budget is tighter by the minute!) and I don't want to screw up.

Is there some way to calculate this in math I can understand?

(current prop: https://ds.morrisdev.com/mo/sharing/ZtUjOoujl)
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Old 13-08-2022, 19:45   #2
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Re: Prop for Electric Motor

You buy The Propeller Handbook by David Gerr and run through the calculations.

It's a bit of a slog, but doable by somebody with a bit of math or engineering background.

The alternatives are to pay the propeller supplier to do it for you, or pay a consultant, or you can take the advice of a random stranger on the internet.

Unfortunately, the selection of a prop is integral with the selection of a drive motor. To do this right you SHOULD have done it as a system instead of picking the motor first independent of the prop. But you got what you got.
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Old 14-08-2022, 10:33   #3
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Re: Prop for Electric Motor

Well, going to a place where people have years and years of experience is not exactly seeking the advice of a "random stranger".

So, random stranger, it seems that you're the only person who's offered advice and I guess I've got book to read.

I'd seen a lot of online calculators, and I figured I'd be able to find the calculation somewhere, but really, it seems like everyone that has a solution does it differently
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Old 14-08-2022, 11:24   #4
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Re: Prop for Electric Motor

Quote:
Originally Posted by coldfish View Post
Well, going to a place where people have years and years of experience is not exactly seeking the advice of a "random stranger".

So, random stranger, it seems that you're the only person who's offered advice and I guess I've got book to read.

I'd seen a lot of online calculators, and I figured I'd be able to find the calculation somewhere, but really, it seems like everyone that has a solution does it differently

I have been playing with electric propulsion for a while. I put it on kayaks, monohulls, and now a trimaran.


What you will find is, yes the larger diameter propeller is better, but there is a minimum clearance before efficiency is lost.

Ultimately you will never achieve a high efficiency with a typical inboard system regardless of what propeller you use, and this is a true shame. It would be great in bursts, to maneuver etc, but not for covering miles.

A typical inboard propeller is 25-40% efficient. This does not count friction losses in the stuffing box, transmission or other losses, only shaft to thrust efficiency. It is simply too small to get enough grip on the water at displacement speeds to do any better. So with the right prop you will be lucky to achieve 40%.

Unless you have twice the batteries and solar that I have, and you probably have more than that... but the point is, I have a propeller that is 65% efficient and it isnt even very good. This is a 32 inch diameter 2 bladed high aspect ratio carbon prop on a boat with a beam (main hull) of 4ft. Using 120 watts it powers the boat about 1.5-2 knots which is slightly more powerful than a sculling oar (which was already sufficient for navigation)

Pedal powered kayaks use a 16 inch diameter propeller. This is a boat just large enough to carry 1 person.

I recommend you continue your conversion with the largest propeller that you can fit and 3 blades is probably better. I would use a lower pitch and a smaller electric motor as well to optimize it for 3-4 knots rather than 6 or whatever it is usually.

Then in addition build a high efficiency outboard which can swing a 60 inch paragliding prop (maybe build two of them) so you can maintain 2-3 knots using solar without battery drain, or at least maintain this speed using half the power of the inboard system. This "outboard" is cheaper to build as it doesnt have more than few kw motor.
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Old 15-08-2022, 09:02   #5
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Re: Prop for Electric Motor

Quote:
Originally Posted by seandepagnier View Post
I have been playing with electric propulsion for a while. I put it on kayaks, monohulls, and now a trimaran.


What you will find is, yes the larger diameter propeller is better, but there is a minimum clearance before efficiency is lost.

Ultimately you will never achieve a high efficiency with a typical inboard system regardless of what propeller you use, and this is a true shame. It would be great in bursts, to maneuver etc, but not for covering miles.

A typical inboard propeller is 25-40% efficient. This does not count friction losses in the stuffing box, transmission or other losses, only shaft to thrust efficiency. It is simply too small to get enough grip on the water at displacement speeds to do any better. So with the right prop you will be lucky to achieve 40%.

Unless you have twice the batteries and solar that I have, and you probably have more than that... but the point is, I have a propeller that is 65% efficient and it isnt even very good. This is a 32 inch diameter 2 bladed high aspect ratio carbon prop on a boat with a beam (main hull) of 4ft. Using 120 watts it powers the boat about 1.5-2 knots which is slightly more powerful than a sculling oar (which was already sufficient for navigation)

Pedal powered kayaks use a 16 inch diameter propeller. This is a boat just large enough to carry 1 person.

I recommend you continue your conversion with the largest propeller that you can fit and 3 blades is probably better. I would use a lower pitch and a smaller electric motor as well to optimize it for 3-4 knots rather than 6 or whatever it is usually.

Then in addition build a high efficiency outboard which can swing a 60 inch paragliding prop (maybe build two of them) so you can maintain 2-3 knots using solar without battery drain, or at least maintain this speed using half the power of the inboard system. This "outboard" is cheaper to build as it doesnt have more than few kw motor.
I've got a 100amp/72v lithium battery, which should get me a fair distance. It's a very heavy boat (13,300lbs displacement) with a full keel, and I'm replacing a 70hp diesel, so I just went with an equivalent power.

Being the insane nerd I am, I even wrote up a little webpage to handle the math end of things here: https://morrisdev.com/eboat/

In reality, the goal here is just to get in and out of the marina under power and, perhaps the occassional 30 minute run back if the tide is against me. Here in SF the wind is almost always there, but the tide can be so strong that you'll be sailing along at a good clip, only to realize that you're going backwards! So sometimes I'd turn on the engine for a bit of a power-assist.

I'm definitely going to have to move up to a 3 blade; I've even been considering a 4 blade. The library had a copy of "The Propeller Handbook" that @ItDepends recommended. I've been taking notes and excluding the Internal Combustion Engine calculations. (power curves, transmissions, etc.. are irrelevant). So far, I've just gotten through 25 pages of "here's how we define torque and how it's related to the 3 different types of horsepower, then crossing them with calculations on electrical systems to find torque in Kw. It's a bit of a slog, but I'll put my notes up on the website and I've already started writing some javascript calculators.

I probably should have tried a canoe or a kayak before I went ahead and dropped a ton of money on this project, but I needed a new motor and I'm starting to feel my age!

No matter what happens, the boat is in the shipyard and I have about a week before they start charging me. So, I'm probably stuck just randomly picking the biggest prop I can fit without losing efficiency. I think perhaps a 3 blade with a very high pitch.
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Old 15-08-2022, 16:21   #6
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Re: Prop for Electric Motor

Quote:
Originally Posted by coldfish View Post
I've got a 100amp/72v lithium battery, which should get me a fair distance. It's a very heavy boat (13,300lbs displacement) with a full keel, and I'm replacing a 70hp diesel, so I just went with an equivalent power.



Being the insane nerd I am, I even wrote up a little webpage to handle the math end of things here: https://morrisdev.com/eboat/



In reality, the goal here is just to get in and out of the marina under power and, perhaps the occassional 30 minute run back if the tide is against me. Here in SF the wind is almost always there, but the tide can be so strong that you'll be sailing along at a good clip, only to realize that you're going backwards! So sometimes I'd turn on the engine for a bit of a power-assist.



I'm definitely going to have to move up to a 3 blade; I've even been considering a 4 blade. The library had a copy of "The Propeller Handbook" that @ItDepends recommended. I've been taking notes and excluding the Internal Combustion Engine calculations. (power curves, transmissions, etc.. are irrelevant). So far, I've just gotten through 25 pages of "here's how we define torque and how it's related to the 3 different types of horsepower, then crossing them with calculations on electrical systems to find torque in Kw. It's a bit of a slog, but I'll put my notes up on the website and I've already started writing some javascript calculators.



I probably should have tried a canoe or a kayak before I went ahead and dropped a ton of money on this project, but I needed a new motor and I'm starting to feel my age!



No matter what happens, the boat is in the shipyard and I have about a week before they start charging me. So, I'm probably stuck just randomly picking the biggest prop I can fit without losing efficiency. I think perhaps a 3 blade with a very high pitch.
You need to read The Propeller Handbook a bit more closely, a high pitched prop is exactly the opposite of what you need.

Your electric hp is no different than ice hp. Use any number of online calculators, using your hp, shaft (prop) rpm, etc. and you will be close.
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Old 19-08-2022, 15:01   #7
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Re: Prop for Electric Motor

Coldfish:

Don't get carried away with the science of it. People have done the hard work for you long, long ago!

https://vicprop.com/displacement_size_new.phpIeds

Put the parameters of your boat into this calculator, and you will get the best answer scientists and "practical men" have been able to devise

For a 32 foot Choi Lee, I would guess that the answer will be something like 18D x 12P.

Then remember that "swept area" which is a function of diameter is what "absorbs" the horsepower the shaft brings to the prop and permits the propeller to covert that horsepower to the "push" that drives the boat forward.

A Choi Lee 32, or any other 27 foot waterline boat, isn't gonna go faster than 7 knots in flat water regardless of how much "push" you put behind it. So more horsepower than required to do that is just a waste.

The prop will turn at an RPM that has an upper limt set by the engine's (motor's) RPM and the applicable reduction ratio. The pitch of the prop tells you how far it will "screw itself forward" in a single turn.

2-bladed propellers on yachts have a "slip ratio"of about 50 % which means that they
only advance half the distance the math sez they should.

The propeller doesn't care whether it's an infernal combustion engine that turns it, or an electric motor.

So use the calculator and your "prop worries" will be over! Then work everything else out on the basis of the prop size the calulater specifies. Don't try to do things the other way about. That will only set you spinning :-)!

All the best

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Old 19-08-2022, 15:24   #8
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Re: Prop for Electric Motor

Quote:
Originally Posted by coldfish View Post
I've got a 100amp/72v lithium battery, which should get me a fair distance. It's a very heavy boat (13,300lbs displacement) with a full keel, and I'm replacing a 70hp diesel, so I just went with an equivalent power.
This seems like a minimal voltage to work with. I would have suggested 120vdc as lots of appliances can run off ac or dc so you could run power tools heater etc without an inverter. You can even run a lot of them at 72v they just spin slower.
Quote:
Being the insane nerd I am, I even wrote up a little webpage to handle the math end of things here: https://morrisdev.com/eboat/
Your numbers are probably off. 101lb drag at 4.9 knot?? You will get more slip than 55% with only a 24 inch propeller.
Quote:
In reality, the goal here is just to get in and out of the marina under power and, perhaps the occassional 30 minute run back if the tide is against me. Here in SF the wind is almost always there, but the tide can be so strong that you'll be sailing along at a good clip, only to realize that you're going backwards! So sometimes I'd turn on the engine for a bit of a power-assist.
I sailed in SF bay and generally you sail hull speed but you can get wind shaded. I think you are going to need to time the tide regardless. Another factor is that normally you can just sail in and out of the marina, in most circumstances. In my mind better to do that to gain the experience.
Quote:
I'm definitely going to have to move up to a 3 blade; I've even been considering a 4 blade.
The 3 blade may vibrate less especially inboard arrangement as both blades get behind the hull at the same time. The other advantage is more efficiency at the particular diameter.

If you wanted maximum overall efficiency, it would generally have fewer blades of much larger diameter.

Quote:

No matter what happens, the boat is in the shipyard and I have about a week before they start charging me. So, I'm probably stuck just randomly picking the biggest prop I can fit without losing efficiency. I think perhaps a 3 blade with a very high pitch.
I think you mean low pitch. This will help optimize for more efficient speeds.

Quote:
Originally Posted by fourlyons View Post
You need to read The Propeller Handbook a bit more closely, a high pitched prop is exactly the opposite of what you need.

Your electric hp is no different than ice hp. Use any number of online calculators, using your hp, shaft (prop) rpm, etc. and you will be close.
Yes it is. The electric has the efficiency over a wide torque and speed range. It _is_ different.

Electric can be used in bursts to assist tacking to to ensure you never lose control while allowing you to sail in and out of the marina. You probably wont even need the motor most of the time if you do it right, but dont have to worry if you make a small mistake as the electric motor can run for a few seconds. diesel cannot do this: it is not the same and not as useful.. diesel ensures you consume vastly more energy instead as it must run for a long time and you dont learn to sail in and out.
Quote:
Originally Posted by TrentePieds View Post
Coldfish:

Don't get carried away with the science of it. People have done the hard work for you long, long ago!

https://vicprop.com/displacement_size_new.phpIeds
This calculation is flawed as there are many more parameters to a propeller besides diameter and pitch. My propellers have a high aspect ratio which improves efficiency but this calculator does not have a parameter to enter this.
Quote:
Put the parameters of your boat into this calculator, and you will get the best answer scientists and "practical men" have been able to devise

For a 32 foot Choi Lee, I would guess that the answer will be something like 18D x 12P.
This is plain wrong. They dont care about efficiency at all, only powering the boat at hull speed.
Quote:
A Choi Lee 32, or any other 27 foot waterline boat, isn't gonna go faster than 7 knots in flat water regardless of how much "push" you put behind it. So more horsepower than required to do that is just a waste.
It is a waste to use the prop you suggested because it will require so much more horsepower to do the same thing as a much larger diameter prop. a 27ft boat should not have 18D 12P. 48D 16P would consume far less energy to propel the boat at all useful speeds.
Quote:
2-bladed propellers on yachts have a "slip ratio"of about 50 % which means that they
only advance half the distance the math sez they should.
The number of blades does not determine the slip ratio. It has more to do with the area the propeller sweeps and other factors.

It slips this much because you are suggesting a tiny 18 inch propeller for a 27 ft boat. This pushes a small stream of water too fast with lots of slip. If you want to achieve 85-90% efficiency it must be a big prop, and the slip may be less than 10% with 2 bladed or 3 bladed propeller.

Quote:
The propeller doesn't care whether it's an infernal combustion engine that turns it, or an electric motor.
Actually... the electric motor can accelerate and decelerate in a single revolution thus improving efficiency slightly in turbulent water with sufficient feedback and an clever control system. I have found some evidence of people doing this sort of thing with special brushless controllers. Something like this is completely impossible with infernal combustion.
Quote:
So use the calculator and your "prop worries" will be over! Then work everything else out on the basis of the prop size the calulater specifies. Don't try to do things the other way about. That will only set you spinning :-)!
Just dont use that calculator unless you want a crappy efficiency like all the other boats.

In my mind it makes no sense to have all those lithium batteries then churn water with them instead of getting the appropriate size prop which in this case is probably at least 60 inch diameter. Yes it may require some engineering to accommodate, but its really nothing compared to living in a world where a few people have huge lithium banks and the rest of people are poor, or a world utterly destroyed. I dont think we can afford to be so wasteful with renewables.

an inboard powered prop powered with an electric motor makes about as much sense as putting sails on a powerboat. It is an expensive way to get a poor result. The worst part is people doing this post their results and others believe that electric propulsion is much more expensive and less efficient than it actually is. So if you do this and are happy with it, sure, ok, but would be good to point out your are slipping so much that the propeller is only 25% (maybe up to 40%) efficient with this particular inboard system, and make it clear to others that they would be unwise to copy this.
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Old 19-08-2022, 16:51   #9
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Re: Prop for Electric Motor

'salright Sean :-) I'm having difficulty relating the concept of "efficiency" to a 32-foot Choi Lee, let alone the notion of fitting a four-foot prop in it's aperture :-)!

Cheers

TP
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Old 19-08-2022, 17:54   #10
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Re: Prop for Electric Motor

Quote:
Originally Posted by coldfish View Post
I...
Being the insane nerd I am, I even wrote up a little webpage to handle the math end of things here: https://morrisdev.com/eboat/
Just out of curiosity I looked at your web page with those calcs.

I noticed that for both 5.9 knots and 6.6 knots you give three different HP calculation each. Hmmm? Still curious. Why the discrepancy?

Second, do you have the torgue and Horsepower curve for you 70HP diesel? That will tell you how much HP your diesel was putting out at various boat speeds.

For example when I was buying a new engine for my 43ft sloop I looked at the torque/horse power curves for my old engine (Westerbeke 50). At 1800hp it was producing 21 HP (we were getting about 6 knots of boat speed). I read Nigel Calder's book which gave a calculation for my boat of 28hp. So instead of a new 50hp engine I bought a 34hp engine and it has been fine. It cruises at 6kt with an 18x18 propellor at 2100 rpm and will push my boat at 7.5 knots into a strong wind and chop at full throttle. So, obviously 70HP was not needed for your boat, more like 35hp I'd guess.

Quote:
Originally Posted by seandepagnier View Post
...It slips this much because you are suggesting a tiny 18 inch propeller for a 27 ft boat. This pushes a small stream of water too fast with lots of slip. If you want to achieve 85-90% efficiency it must be a big prop, and the slip may be less than 10% with 2 bladed or 3 bladed propeller...
Sean, that is a myth that using an 18" prop on a 27ft boat "would push a small stream of high velocity water too fast with lots of slip".

At moderate speeds (low propellor RPM) there is NOT a high speed stream of water jetting back from the prop.

On our boat we've actually observed our propellor in those conditions and from the lack of turbulence or cavitation it appears there is very little slip going on. Calculating the engine rpm, reduction gear, shaft rpm and pitch, when we get 6.0 knots out of our 2100RPM engine (20HP at output of marine gear) it seems like a very efficient operation. The slip is about 50%. Probably a 48" or 60" high aspect ratio propellor would do better but practically speaking I don't think that solution is appropriate for any boat in actual use (for many reasons)...

We have a 18x18 two bladed folding propellor.
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Old 19-08-2022, 20:04   #11
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Re: Prop for Electric Motor

Quote:
Originally Posted by coldfish View Post
I've got a 100amp/72v lithium battery, which should get me a fair distance. It's a very heavy boat (13,300lbs displacement) with a full keel, and I'm replacing a 70hp diesel, so I just went with an equivalent power.



Being the insane nerd I am, I even wrote up a little webpage to handle the math end of things here: https://morrisdev.com/eboat/



In reality, the goal here is just to get in and out of the marina under power and, perhaps the occassional 30 minute run back if the tide is against me. Here in SF the wind is almost always there, but the tide can be so strong that you'll be sailing along at a good clip, only to realize that you're going backwards! So sometimes I'd turn on the engine for a bit of a power-assist.



I'm definitely going to have to move up to a 3 blade; I've even been considering a 4 blade. The library had a copy of "The Propeller Handbook" that @ItDepends recommended. I've been taking notes and excluding the Internal Combustion Engine calculations. (power curves, transmissions, etc.. are irrelevant). So far, I've just gotten through 25 pages of "here's how we define torque and how it's related to the 3 different types of horsepower, then crossing them with calculations on electrical systems to find torque in Kw. It's a bit of a slog, but I'll put my notes up on the website and I've already started writing some javascript calculators.



I probably should have tried a canoe or a kayak before I went ahead and dropped a ton of money on this project, but I needed a new motor and I'm starting to feel my age!



No matter what happens, the boat is in the shipyard and I have about a week before they start charging me. So, I'm probably stuck just randomly picking the biggest prop I can fit without losing efficiency. I think perhaps a 3 blade with a very high pitch.

Propeller matching is remarkably difficult and I once had a yacht with a Hundested variable pitch propeller operated manually via a small crank wheel from streamline through to forward then on into reverse and once it was in forward there was required an ultra delicate adjustment to get the pitch correct. Too much ( by ⅛ of a turn) would pull the revs down dramatically.
There was a propeller produced for motor boats in Australia ( Ballina) called ďthe Church RingĒ and the guy who made em gave me 2 to try out with different HP engines as a market acceptance effort. I only tried one, driven by a Lehman Peugeot through a Hurth mechanical gearbox and it provided a huge change in performance, we even did a home made bollard pull test for before and after results to verify the difference. The problem arose when berthing because the prop would stall the engine going from astern to ahead from what I think was the momentum of the ďringĒ so we didnít adopt them and the company faded away. I thought at the time no yachtsman would want the extra drag so that was another negative but the increase in thrust was definitely established.
Could be an idea for an electric propulsion installation. [ATTACH]263142
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Old 20-08-2022, 11:58   #12
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Re: Prop for Electric Motor

Ducted fan design

Works great if you donít expect to reverse it in a short time period
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Old 20-08-2022, 20:21   #13
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Re: Prop for Electric Motor

Kort nozzles been in use for over a half century, increase efficiency and thrust .
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Old 20-08-2022, 21:18   #14
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Re: Prop for Electric Motor

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Originally Posted by AKA-None View Post
Ducted fan design

Works great if you donít expect to reverse it in a short time period
Or if you don't expect to sail. Ducts provide a net increase in efficiency when powering at slow speeds but they are an efficiency loss when sailing due to drag.
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Old 20-08-2022, 21:21   #15
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Re: Prop for Electric Motor

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Originally Posted by NPCampbell View Post
Or if you don't expect to sail. Ducts provide a net increase in efficiency when powering at slow speeds but they are an efficiency loss when sailing due to drag.


Ducts work fine at high speeds as long as you power the blade inside :-)
Which hopefully you donít do while sailing
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