I'm under propped. What should I move up to?

[Bunji]

Along side hull speed being your biggest limiting factor (motoring) the next problem u have is the props are not deep enough in the water, the reason I know this is because evinrude only do a 15 and a 20 inch shaft length and no high thrust gearbox, Those motors are the worst possible choice the best choice would be the newly released 25 hp high thrust fuel injected yamaha. The high thrust gearbox ratio allows u to run a bigger prop. Cruise all day on one engine 6 knots (super efficient) or use both 10 possibly 12 knots ( not very efficient)

[wsmurdoch]

In the early 80s when my Tanzer 22 was a new boat, we wanted to make a long trip on the ICW. I wanted the best fuel economy from its 7.5 hp Johnson outboard and 3 gallon fuel tank. I also wanted good thrust in reverse to stop the boat in a slip. I got all kinds of conflicting advice on propellers, engine rpm, and boat speed. So..

I got all three OMC propellers that would fit the motor. That way I had all of my possible propeller choices. I bought a 3 ft length of 1/2" clear plastic pipe, hooked it to an outboard fuel hose, and with the pipe vertical and taped to the backstay, I marked it in ounces. That, a funnel, and a small jar of gasoline let me quickly measure fuel consumption. I got a hand held mechanical tachometer to measure engine rpm. I got a spring scale to measure the engine thrust when tied to the dock. The boat had a Signet knotmeter to measure boat speed. I set to work in windless flat water on our lake.

Generally what I learned for my boat and my engine was... 1) A slower boat speed uses less fuel per mile. 2) The high thrust propeller does produce more static thrust, and, perhaps because of the exhaust being pulled through the propeller in reverse, it produces significantly less reverse thrust than forward thrust. 3) At equal boat speeds, the high speed propeller used slightly less fuel per mile than the other two.

I put the standard propeller back on the engine and have not worried about it since. But, I still have all the stuff in the garage.

Bill

[Chotu]

Originally Posted by Bunji

Along side hull speed being your biggest limiting factor (motoring) the next problem u have is the props are not deep enough in the water, the reason I know this is because evinrude only do a 15 and a 20 inch shaft length and no high thrust gearbox, Those motors are the worst possible choice the best choice would be the newly released 25 hp high thrust fuel injected yamaha. The high thrust gearbox ratio allows u to run a bigger prop. Cruise all day on one engine 6 knots (super efficient) or use both 10 possibly 12 knots ( not very efficient)

Maybe selling the Etecs and putting that toward the new Yamahas could be a way to go.

A picture is worth 1000 words. Here is what the outboards look like.

[Chotu]

Originally Posted by wsmurdoch

In the early 80s when my Tanzer 22 was a new boat, we wanted to make a long trip on the ICW. I wanted the best fuel economy from its 7.5 hp Johnson outboard and 3 gallon fuel tank. I also wanted good thrust in reverse to stop the boat in a slip. I got all kinds of conflicting advice on propellers, engine rpm, and boat speed. So..

I got all three OMC propellers that would fit the motor. That way I had all of my possible propeller choices. I bought a 3 ft length of 1/2" clear plastic pipe, hooked it to an outboard fuel hose, and with the pipe vertical and taped to the backstay, I marked it in ounces. That, a funnel, and a small jar of gasoline let me quickly measure fuel consumption. I got a hand held mechanical tachometer to measure engine rpm. I got a spring scale to measure the engine thrust when tied to the dock. The boat had a Signet knotmeter to measure boat speed. I set to work in windless flat water on our lake.

Generally what I learned for my boat and my engine was... 1) A slower boat speed uses less fuel per mile. 2) The high thrust propeller does produce more static thrust, and, perhaps because of the exhaust being pulled through the propeller in reverse, it produces significantly less reverse thrust than forward thrust. 3) At equal boat speeds, the high speed propeller used slightly less fuel per mile than the other two.

I put the standard propeller back on the engine and have not worried about it since. But, I still have all the stuff in the garage.

Bill

Ha ha ha. Thanks, Bill. That last paragraph really put a smile on my face.

[rslifkin]

That's definitely an option. I'd probably talk to Evinrude first, and then if it looks like the answer is different outboards, talk to Yamaha and others to figure out which model is likely to be your best bet.

A Kort nozzle type shroud around the props might also help. I know there are some off the shelf ones sold for outboards. They typically increase thrust at lower speeds, so it might be another piece of the puzzle to get those hulls pushed efficiently.

[Rucksta]

rslifkin

Great idea

The off the shelf models typically produce a 15-20% increase in thrust.
A well designed unit - matched to prop and hull form can produce 50%

The basic models like the MACs (more of a prop guard than a duct) can be improved by welding a 3/4 -1" tube around the entry.

For $134 and drilling 5 holes I reckon I would try one and if it worked do the other engine.

[a64pilot]

Originally Posted by Chotu

Obviously I understand the difference between a power cat hull shape and a sailing cat hull shape.

This still doesn't explain it, but Grit and others talking about the props being undersized did explain it.

It's thrust. That's it. Props don't have enough surface area to create enough thrust to move the boat past 8 knots because they slip/cavitate due to small surface area relative to what they're pushing.

Hull shape isn't the reason.. Thrust is.

Nothing to do with squatting. I don't squat because I don't have weight aft. +1 for outboards despite all the -1s we are talking about in this thread.

If I put an imaginary enormous saildrive well ahead of amidships with super huge propellers with lots of power behind them, I could do 20 knots under power with this boat in displacement mode. Fact.

The reason I'm having problems as found out here is very low surface area of the props so very low thrust.

Increasing surface area (or backing off to a speed I don't slip/cavitate at) is the only solution to this problem. Your boat has an entirely different issue if it squats.

This is some pretty bad news for me if I want to motor up the icw. Makes it very expensive because the energy is going into creating the cavitation. Either that or I have to go unbearably slow.

Now your understanding, so far as going unbearably slow, you will do doing the same speed the rest of us do, maybe a little faster so you will have company. Actually a boat that runs say 10 kts in the ICW is a pain for themselves and everyone else, constantly slowing down and speeding up to pass people where if you run 6 or 7 kts, it’s a steady cruise for you and everyone else.
I think your speed expectations are possibly a little optimistic for this boat.
I believe that in fact you would actually motor at least as fast and with less fuel burn and noise and weight etc with whatever the biggest actual high thrust motors you can find, that may be 15’s but I’m not sure.
I would not be cutting up the gear case, that could have very bad unintended consequences and if it worked well then I’d bet modified ones would be for sale.

If your motors are new you may can swap them for high thrust ones.

On edit, for almost any motor there is from a fuel burn perspective usually the middle third of the RPM range is where you want to be, now you will burn less at lower RPM, but the middle third is usually a good compromise between noise, vibration, speed and fuel burn, and engine life

[CF32907]

Originally Posted by a64pilot

Now your understanding, so far as going unbearably slow, you will do doing the same speed the rest of us do, maybe a little faster so you will have company. Actually a boat that runs say 10 kts in the ICW is a pain for themselves and everyone else, constantly slowing down and speeding up to pass people where if you run 6 or 7 kts, it’s a steady cruise for you and everyone else.
I think your speed expectations are possibly a little optimistic for this boat.
I believe that in fact you would actually motor at least as fast and with less fuel burn and noise and weight etc with whatever the biggest actual high thrust motors you can find, that may be 15’s but I’m not sure.
I would not be cutting up the gear case, that could have very bad unintended consequences and if it worked well then I’d bet modified ones would be for sale.

If your motors are new you may can swap them for high thrust ones.

On edit, for almost any motor there is from a fuel burn perspective usually the middle third of the RPM range is where you want to be, now you will burn less at lower RPM, but the middle third is usually a good compromise between noise, vibration, speed and fuel burn, and engine life

CaptVR here,
The sailor above is quite rite, after making the trip from fl. to new england a half dozen times, there are some areas and sound you can go any speed, 2/3 the trip on the ICW, you are required to go slow or have timed bridges. Putting that aside, you will get a dozen different answer's, this is not the place to find the size prop for your boat. Go to a dedicated prop shop, give them the stats and they will give you there best advise, and they are the ones educated in prop selection and or modification. They will give you the best answer because there rep depends on it and they don't want to see your smiling face again until you have a different dilemma. The main reason to have the right spec prop, under propped you go slower and burn more fuel, over propped you build deposits in the cylinder faster because all fuel is not being burned and you will where out the motor sooner, and again your wasting fuel.. E-Tech shops have very expensive computers that have algorithm's that will give the best prop for the vessel. Take advantage of their expertise. Capt. Vince Rakstis, Ret MS

[garyfrankovich]

Have you considered calling on a Prop-Shop for advice?

[rreid01]

I'm curious about the boat. Any chance you can post some pictures?

I have a 30' Motorcat--also a powered catamaran. Ours has a pair of older 50hp Hondas. We are currently in the process of re-powering with new 60hp FI Hondas.

Also, I had good luck working with Turning Point Propellers choosing the propellers for my new application. They have a Prop Wizard application on their site, but for applications like this you can call them and they will work with you to come up with a suggestion. They will also take the props in return for a different size if they don't work out.

[Cliff Meima]

Please read this.

https://irp-cdn.multiscreensite.com/...mancePower.pdf

Your welcome.

[CaptWho]

I haven't gone through all the messages in this thread, but has anyone mentioned Propeller Calculator / Prop Calculator?

[Buzzman]

Interesting, that info from Schionning. A 40' cat with 125HP motors required to achieve 21kts. You're 100HP shy, there, chotu..!! lol

As has been noted, the problem is thrust, and you ain't got enough of it.

If, as someone suggests, Evinrude don't do a lower-geared, high-thrust bottom leg, then you're pretty much buggered. You need to change motors. Or get used to travelling slower...

I was going to suggest a 'whale tail' to help reduce cavitation, but it appears from your pics that the motors are in wells...??? Or is that just the angle of the photo?

Normally on a cat motors are beside the inside face of each hull. Or hung off the rear of the aft beam, and so not protuding out from under the hull like a Saildrive or shaft-driven prop.

It might be that the closeness of the lower underside surface of the hull is affecting the cavitation, somehow 'tearing' the laminar flow off the surface of the hull....but hey, I'm no aero engineer and that could be complete crap....but 'steps' in hull design have been shown to produce 'ventilation' so a well might be doing that, which could be exacerbating the issue of low thrust. For example, it's well known in fast sailing cat circles that a centreboard 'well' creates issues, which is one reason hi-=po cats use daggerboards, that have a much small, tighter, 'well', for less disruption of the laminar flow under the boat.

But I do know that with smaller outboards, a large whale tail can be of use in reducing cavitation and providing a small amount of additional thrust.

FTR, the 'whale tail' plate built into the lower leg of the outboard is more properly called an 'anti-ventilation-plate' rather than an 'anti-cavitation' plate.

Power boat guys know all this stuff, but is is still a 'common misuse' to refer to 'anti-ventilation' as anti-cavitation.
While the one can lead to the other, they are different.

Ventilation most often appears when the prop pops out or gets too close to the surface, and the motor loses thrust and the revs rise, like a slipping clutch in a car going up a hill. Basically it 'sucks' air into the laminar flow over the prop blades, reducing 'grip'. Like a car wheel spinning on a muddy road surface. If you back off the power, you regain 'grip'.

What you have is more likely to be just cavitation, which occurs as the pressure drops at the back of the blades, and the water boils in the lower pressure environment, but a similar result is that the thrust is lost as the blade is, effectively, 'slipping' like a clutch and no longer 'pushing' at the water.

The answer to this is to use less revs.

To maintain the thrust at those lower revs you need a different prop (either bigger or 'effectively bigger') OR you need a bigger motor that also has a bigger prop to start with.

Lack of sustainable thrust is the problem.

Most likely, the reall issue is the incorrect choice of motor for application in the first instance, and no amount of mucking about with props will make a hill of beans difference.

In the end you may simply have to save up for a new set of high-thrust Yamahas. Or resign yourself to slower speeds under power.

Hopefully Evinrude techs can assist in a prop choice that may 'improve' the situation..a bit...

Let us know how you get on.

[Old fella]

Originally Posted by a64pilot

It’s very likely that you won’t be able to prop the boat correctly as you can’t get a big enough prop and or turn it at a low enough RPM to be proper. Because you can’t correctly prop a displacement boat with a regular outboard is why high thrust outboards exist, and even then they are very much compromises of what can be done with an outboard.
I assume your motors are not high thrust ones?
I believe that no matter what prop you put on the boat the motor will cavitate it at high power if it’s not a high thrust one.
Your best option to save money is to slow down and run the boat at a slower speed, it’s not a hull limitation, but a motor one, but it’s a limit none the less.
Try to find out the efficient motor RPM, for example my little Suzuki will go into “lean burn” between 3000 and 5000 RPM, so I need to stay in that range.
You can also graph an RPM vs speed chart and based on that try to find the RPM at which prop slip becomes excessive and stay below that point. That is I think the best route, prop slip of course is wasted energy, and the energy source is the fuel tank.
With no income I assume money not spent is more important than speed, so just accept a slow speed for money saved.

It may be that cruising on one engine will burn less fuel

I couldn't agree with this more, I don't know if this helps but the chap that does my props when needed told about 25 years ago, a rough rule of thumb was with 2 to 1 reduction 1 inch equates to about 400 rpm, good luck.

[mcarthur]

Originally Posted by Chotu

Check your math.

2.84 x Sqrt(lwl)

Standard formula for this type of Catamaran.

Sqrt 50 = 7.071
2.84 * sqrt 50 = 20.08 knots. Exactly the speed of these boats under sail.

Very interesting - where did the 2.84 come from?

The number is called the SL ratio. Dave Gerr has some interesting notes about it, and believes the maximum SL ratio for a vessel is

8.26/power(DL ratio,0.311).

The DL ration (displacement-length) is

Displacement in long tons (2240lb) / power(0.01 * LWL in ft,3)

which would be an SL of about 2.4 for you (assuming weight is 7 long tons and 51' LWL?) rather than 2.84. Still extremely high, giving 17 knots! And of course it says nothing about how much power is required to get to that speed