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Old 11-10-2020, 14:44   #31
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Re: Pearson 30 atomic 4 optimal rpm at hull speed?

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Originally Posted by Scubaseas View Post
I would bet more than a few cold beers to guess if you tossed 80 HP at it you still could not get a P30 to get up on a plane. I get the whole climbing the bow wave thing.
You could throw 200 HP at it, it still wouldn't plane.
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Old 11-10-2020, 16:38   #32
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Pearson 30 atomic 4 optimal rpm at hull speed?

Sooner or later it will flip on its side and plane on the topsides, keel, and rudder.
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Old 19-10-2020, 09:48   #33
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Re: Pearson 30 atomic 4 optimal rpm at hull speed?

You can go faster over ground than hull speed by surfing or with a current. But it is a physical impossibility to go any faster through the water than hull speed because in order to do so you have to get on plane which is not possible. To understand, if you sre set to east by 5 knot current and you steer east at 6 knots your speed over ground is 11 knts east but your speed through water is inly 6 knts east
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Old 19-10-2020, 09:57   #34
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Re: Pearson 30 atomic 4 optimal rpm at hull speed?

If you are converting to electric drive then work with the supplier to determine if you need to gear refuce or increase prop diameter/pitch.It will all depend upon the type of motor and controller you choose relative to the size and displacement of your boat.
For example, I converted from Yanmar3YM30 to a 20kW PMAC 3 phase ac motor. I used a 3:1 gear reducer because that gave me a top rpm of shat around 1400 RPM and allowed reusing existing prop. But if you use a dc motor or an ac induction motor you will have different speed/performance specs.
Any of the DIY suppliers can provide guideance google them and use their web sites
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Old 19-10-2020, 10:04   #35
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Re: Pearson 30 atomic 4 optimal rpm at hull speed?

With regards to the hull speed being the maximum speed for the boat, the Naval Architect I work with says that is not true. You can exceed hull speed but the cost is additional power required is very large.


Try this explanation on Hull Speed from a naval architecture consulting group.


https://dmsonline.us/the-truth-of-hu...g-speed-limit/
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Old 19-10-2020, 10:11   #36
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Re: Pearson 30 atomic 4 optimal rpm at hull speed?

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You can go faster over ground than hull speed by surfing or with a current. But it is a physical impossibility to go any faster through the water than hull speed because in order to do so you have to get on plane which is not possible. To understand, if you sre set to east by 5 knot current and you steer east at 6 knots your speed over ground is 11 knts east but your speed through water is inly 6 knts east
Hull speed is a theoretical maximum.
Think about your own statement, to surf by definition you have to climb over your bow wave and your speed through the water therefore exceeds your 'hull speed'. This has nothing to do with speed over ground.
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Old 19-10-2020, 11:00   #37
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Re: Pearson 30 atomic 4 optimal rpm at hull speed?

You obviously didn’t read my entire response. Speed over ground and speed through the water are very rarely tge same thing when sailing. I have indeed surfed down waves in Lake Erie and in the Exuma Sound where boat speed over ground exceeded my hull speed but thats was because the water was moving about 6-7 knots in addition to moving through the water near hull speed. If the water isn’t moving I doubt youll ever get a displacement hull above hull speed regardless of power but I guarantee you can get near instant acceleration of the boat and almost twice hull speed on a 10 foot wave overtaking you from the stern
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Old 19-10-2020, 11:10   #38
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Re: Pearson 30 atomic 4 optimal rpm at hull speed?

I was crew on a Catalina 22 in a race on Lake Monroe near Sanford Florida several years back. If I use 19.33ft for the LWL I calculate a Hull Speed of 5.89 knots. We had a speed log puck mounted on the mast facing the cockpit. It started out pretty breezy at 12 to 15 MPH wind and over the course of two laps around the course worked up to 30 MPH sustained with guts over 35 MPH. They called the race due to conditions and we were on the final downwind leg headed for the finish line so we could get off the course. The speed puck registered over 13 MPH (I am pretty sure it was set for knots but I will concede I can't prove that so I will go with the lower speed MPH). It is a shallow inland lake so no currents. We were absolutely planing, wake coming together 10-15ft behind the boat, Savitsky as my witness! Sails set wing and wing (storm jib with no reefs in the main).
If I convert from 13MPH to kts (since I can't prove the puck was set to knots) I get 11.29 kts. Clearly over hull speed. Yes we were greatly over powered, yes we were using the two crew as moving ballast to keep our stability and yes we were planing. Yes we were really lucky we didn't break anything.
I have had the pleasure of being part of the Gunboat 55 design team and have been out in one of the boats running 25+ kts on a broad reach. Hull speed for them is roughly 10 kts. By definition we were planing.

Moral of stories, with enough wind power and the right boat hull you can exceed hull speed cleanly.

Can you exceed hull speed with engine power, yes, with enough power to get over the planing hump you can make just about anything plane. It may be very difficult and expensive, but many boats can be made to plane. There will be hulls where you don't have the room to hang or install enough horsepower to make them plane I am sure and there will be some whose bottom friction of lack of any lifting surfaces makes planing unobtainable.


The question in my mind is why would you want/need to exceed hull speed on a sailboat in the first place? Need to go faster, buy a longer boat with enough power to run at or close to hull speed.
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Old 19-10-2020, 14:22   #39
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Re: Pearson 30 atomic 4 optimal rpm at hull speed?

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You obviously didn’t read my entire response. Speed over ground and speed through the water are very rarely tge same thing when sailing. I have indeed surfed down waves in Lake Erie and in the Exuma Sound where boat speed over ground exceeded my hull speed but thats was because the water was moving about 6-7 knots in addition to moving through the water near hull speed. If the water isn’t moving I doubt youll ever get a displacement hull above hull speed regardless of power but I guarantee you can get near instant acceleration of the boat and almost twice hull speed on a 10 foot wave overtaking you from the stern
and you obviously haven't read the whole thread. I was running down the face of smooth 20ft rollers with a tail wind approaching 40 knots. My knot log read 12+ knots.
It's not all that hard to exceed hull speed when you have an open mind. :>)
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Old 19-10-2020, 16:16   #40
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Re: Pearson 30 atomic 4 optimal rpm at hull speed?

Apples and oranges. You will want a different prop, probably a fixed one, as big as you can swing and still have an inch and a half to two inches hull clearance, pitch approximately equal to the diameter.

Now that you know what prop you want, and presumably what motor you want, what voltage and what SUSTAINED max power rating, you can figure out what reduction ratio you want.

TIP: an enclosed gearbox is nice, but if you go with belt drive you can quickly and cheaply change the reduction ratio by changing one or both pulleys. There are lots of formulae and spreadsheets for calculating this stuff but in my experience as an electric boat guy, you will want to do some tinkering and fine tuning to get it optimized.

That boat sounds like a good candidate for EP. Any new diesel will cost more than the boat is worth even after installing the diesel. A used diesel is someone else's old problems, usually. Your boat is small enough to push easily with a 10kw to 12kw motor and a 48v system will work nicely.

But meanwhile, Moyer Marine has rebuilt A-4s and you can now get new cast blocks. When you do a couple of simple upgrades to the carb, install an electric fuel pump, and add electronic ignition, it's not such a bad little motor as long as you have plenty plenty of forced ventilation to take care of any gasoline vapors.

Upgrading to electric propulsion is going to cost you, including batteries, a bare minimum of about $2600 and that is sourcing parts yourself, mix and match, doing your own engineering which is no small task. A pro install including batteries will probably cost about $6k for any decent amount of range at all. A rebuilt A4 will be an easy drop in replacement and cost more than what I paid to repower my Cal 2-27 but less than a professional turnkey EP install. Something to think about. If you motor a lot, consider another Atom Bomb. If you don't motor very much, you will probably like electric. If the boat was newer and more valuable, I would go with a new Beta diesel.
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Old 21-10-2020, 08:23   #41
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Re: Pearson 30 atomic 4 optimal rpm at hull speed?

I have a Maxi 87 (25' WL, 6.8 knot hull speed) and will replace my Volvo MD6A with electric, probably a 12-18 KW 48V Thunderstruck with toothbelt reduction gear.
I'm considering an Autostream 3-bladed feathering prop.

So, guys, please : Any suggestions or even experiences of large prop diameter and rpm ?

OP, before deciding a conversion to EP, please be clear if you want to SAIL your boat, or go long distance motoring whenever you like.
The range is limited by your battery bank, unless you add a (portable?) genset.
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Old 22-10-2020, 16:19   #42
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Re: Pearson 30 atomic 4 optimal rpm at hull speed?

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I'm considering an Autostream 3-bladed feathering prop.

So, guys, please : Any suggestions or even experiences of large prop diameter and rpm ?
I'd say definitely consider regen capability for your electric setup when choosing prop. Using your electric motor as a hydro generator while sailing to supplement the energy needs for your boat is a good way to compensate for the reduced runtime compared to an IC engine & it's especially nice for sans solar conditions (nightsailing, rainy weather)

I was planning on keeping the original 2-blade on my p30 because I'm cheap but after many suggestions in this thread and other places I'm hearing 3-blade fixed is better for regen and efficiency so it is the way I'm planning to go.

I'm also unfortunately finding out that in order to maximize both efficiency and regen capability in an electric system you really have no choice but to gear down, which means throwing any top end thrust out the window (note: this would be for safety factory not for going fast but for dealing with scenarios such as strong opposing current/headwinds or accidental grounding where you need extra thrust to try and back your keel out the mud)

I guess I could always find a shop to fab a custom transmission with both a 2:1 and a 1:1 gear in it instead of fwd/rev, that way I might still be able to have the full prop rpm range in a tough situation if I ever needed it, but the cost would probably outweigh the benefit by far.
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Old 22-10-2020, 20:09   #43
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Re: Pearson 30 atomic 4 optimal rpm at hull speed?

Thanks, Tackrun

My idea to mount a powerful (12-18KW) motor with a big prop is to have a lot of torque in an emergency, like you described :-)
For general use, I will of course not use all this potential power, and let the setup run with low rpm. That's why a big prop may be more efficient. Low rpm = no cavitation.

Many electromotors run (at peak) 5000 rpm, so a 3:1 or 5:1 gearing may be useful for the big prop, depending on the size.

I could convert my Volvo Saildrive 100S, but maybe better to change to a new shaft drive, making the high torque possible.

What do you mean by "throwing any top end thrust out the window" ?
For an emergency (I AM sailing in silt/sand bottomed shallow waters) I will need thrust (high torque) to spin the big prop, not high rpm.
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Old 22-10-2020, 20:33   #44
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Re: Pearson 30 atomic 4 optimal rpm at hull speed?

[
Quote:
Many electromotors run (at peak) 5000 rpm, so a 3:1 or 5:1 gearing may be useful for the big prop, depending on the size.

I could convert my Volvo Saildrive 100S, but maybe better to change to a new shaft drive, making the high torque possible.
I doubt if one can fit 3:1 gears (let alone 5:1) inside the gear case of any saildrive. Just not enough room for bigger spur gears in there. And the cost of custom gears would be pretty high.

If you were to design a drive using toothed belts it could be possible to arrange a two speed drive utilizing differing size drive pulleys. dunno how practical that would be for you, though.

And finally, I'd reconsider the prop. Feathering props are inherently inefficient due to their flat, untwisted blades and with the limited power of the electric drive you don't want to be throwing away efficiency. Further, most feathering props are ill suited to regeneration because they, well, feather when the shaft is stopped or retarded in rotation... that's the whole point of them!

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Old 22-10-2020, 23:02   #45
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Re: Pearson 30 atomic 4 optimal rpm at hull speed?

Most of the motors commonly used for EP in small sailboats can be operated at up to or nearly up to 5k RPM without self destructing. That doesn't mean you should use that motor speed as your top end speed. Normally you use a constant that is unique to that motor, multiplied by nominal system voltage, for your optimum top speed in terms of efficiency. Your ideal top speed is more likely to be around 3000 to 3600 RPM. With a given hull and displacement and a given prop diameter, pitch, blades, and general blade profile, and with a given desired top speed, and with a desired top operating speed through the water, (typically a large fraction of calculated hull speed unless you need range more than you need speed) you can then find your initial reduction ratio. You don't base it on the prop speed you are familiar with or just like, and the motors max RPM rating. You have to fight for every tiny bit of efficiency you can get.

You can put together a system that will let you motor slowly all day, or "in the corner" for about an hour, pretty easily, if you do it right. You can also design a system that will give you a single digit max range and 20 minutes at max speed, for the same money. I did my homework for two years before I ever started buying components, and I suggest you do the same or more. Or go with a turnkey system professionally installed, for so-so but not too bad efficiency and 3x the cost. If you are gonna DIY it, you need to know what you are doing or you will be expremely disappointed. With a diesel or gas engine you have plenty of energy to throw away and you will hardly notice that you are using 15% more fuel than you should. With EP, every percentage is important. You can't afford to throw away watts, much less kilowatts.

Biggest thing... install the biggest bank you can physically fit in the boat, unless you are mostly day sailing and just need power for docking and maneuvering. If you go with FLA, I strongly suggest GC-2 golf cart batteries if you are on a budget, a DIESEL if you are not. LiFePO4 if you are just an EP fanboy and want the latest and greatest, lightest, most energy dense, and most expensive, and you truly understand the chemistry and how to use it safely. Don't use 12v batteries unless they are Rolls or similar TRUE deep cycle. You don't want to limit yourself to 40% max drawdown. Golf cart batts or Rolls-Surrette can stand pretty deep discharges and boat motion won't bother them.

My basic rule of thumb:
You can do hull speed with EP
You can have fairly good range with EP
You can operate with a fairly small bank with EP
You only get one of the above. Whichever you pick, kiss the other two goodbye.

My suggestion is get used to the idea of motoring at around 2.5kts which seemed to be sort of a sweet spot for my Cal 2-27. Yeah I hat the torque and I could dig a big hole in the water if I wanted to show off in the marina and wasn't worried about twisting my shaft in two, but normally I was satisfied to go nice and slow. Slow as in my speed under sail in light airs.
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