How do I estimate prop & HP requirement?

Discussion in 'Boat Design' started by Deering, Jan 21, 2013.

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DeeringSenior Member

I'm playing around with the idea of lengthening the hull of my aluminum catamaran from 32 ft to 51 ft. This project would include repowering the boat.

I've modeled the new hull in Delft, then run that design through Michlet. Michlet gave me resistance curves.

So now I know the resistance I need to overcome at a given speed. How to turn that into horsepower and prop size? My thinking is that I need to generate enough much thrust with the propeller at that speed to overcome the hull resistance (along with wind and other resistance). So if I select the appropriate diameter and pitch prop that gives me the necessary thrust at a suitable RPM, I then mate an engine/gearbox to that prop. Basically work my way backwards up the drivetrain.

I went to Dave Gerr's prop book and found his thrust formula for a prop at speed (page 60), and solved it for BHP. Problem is, the result I'm getting for my designed cruising speed is far below what I had expected - like about 1/3 what I expected. I'd be overjoyed if that was the case, but I think I'm doing something wrong.

Is there a better way to convert Michlet resistance curves to engine and prop sizing estimates?

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gonzoSenior Member

Dave Gerr's system is quite adequate for mid to low performance vessels. Your boat falls in that category. If you lengthen the hull without adding any or much beam, the resistance at low speeds probably decreased.

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DeeringSenior Member

That's what I was expecting to see.

My design lengthens the waterline by 50% with no increase in beam, a reduction in draft, and little increase in displacement.

Instead, I saw no drop off in resistance at my target design speed of 15 kts, and in fact the resistance rose, based on Michlet. But when I work through Gerr's thrust formula to solve for the HP, it gives me a far lower value than I'm using to power the boat now.

Based on what I'm seeing in Michlet, I'll have to give up on the project. A 50% increase in WL and the drag increases...that wasn't in the plan.

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Before you can use any software you need to validate it. Thus, you know what your existing boat can do...so, run it through Michlet and run the numbers. If the numbers you get on your existing boat are at variance with the reality of what it is, then you have a problem. That being the program is being used beyond its limitations, or more data is required, or both!

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DeeringSenior Member

Yes, I have.

The resistance curves are about what I expected to see. And I know what the power demands are at those speeds. Using Michlet and Gerr's method, my current boat should require 50 hp to run at 15 kts...reality is around 200 hp to do that. And Michlet says the redesigned hull requires even more power, but Gerr's formula yields 60 hp.

Incidentally, I incorporated the hull shape advice you provided a couple months ago.

Question, because this is a planing hull that I'm trying to convince to be a displacement hull, the beam/draft ratio is quite large - about 2.8 - could this be making this prospect pointless?

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So this already suggests that something is wrong. If you current boat uses 200Hp, yet the "calculations" suggest only 50Hp...something is amiss!

That is beyond the safe limits of Michelt. Thus whatever "numbers" your getting they are just that...numbers!

I know Leo is making the L/B limits lower, but it is ostensibly for higher L/B ratio hull forms. So if you're outside the range, chances are the numbers wont mean much. Other than pretty pictures.

I vaguely recall something. How did it go...better, worse, same??

For you current boat, what is the waterline length and the displacement at full load and the total Hp you have, and what's the flow into the prop like?

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DeeringSenior Member

Yes, the pictures are pretty, no doubt about that. Thank you for cluing me in about the Michlet limits - if it was in the manual I missed it.

Yes, your hull suggestions were very helpful. Thank you. But given that the results are not looking promising regardless...

Current WL is 9.8 m. Full load displacement is about 14.5 tonnes. Flow to the prop is very good though there are some hull features forward of the props that likely cause some turbulence.

Design WL for the modified hull would be 15.5 m, similar displacement, and flow to prop will be good, though with a skeg forward of it. Hull would be cleaned up to reduce turbulence.

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These are very similar dimensions as a small cat I did last year.

The trouble is, the L/D ratio is poor, yours for the 9.8m on 14.5 tonne is just 4.0.
The one I did was 9.4m weighing at some 9-9.5tonne, the L/D ratio is 4.5, so "similar"; since both very heavy for their lengths.

Here is the rough Hp/tonne graph I used to estimate the speed, based upon short fat cat hull forms. The sea trials results were just about spot on too; only 0.2knots difference.

As a rough guess, this should suffice.

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Leo LazauskasSenior Member

I have extended the capability to hulls with L/B as low as 4, but it is not in Michlet. I haven't done much with Michlet for several years now.

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DeeringSenior Member

Yes, that's pretty similar to my current boat. But if I increase the length by 50% the L/D becomes much better, right?

What happens if you increased the boat length above by 50%?

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DeeringSenior Member

Leo, what are the the beam/draft limitations on Michlet, if any? My hulls are a bit wider and shallower than a typical displacement cat. I noticed that Michlet yielded a very large divergent wave resistance number and was wondering if the D/B ratio was the reason.

Is there a more suitable tool I should be using out there? Thanks...

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Roughly, as below:

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Leo LazauskasSenior Member

There are no fixed limits on L/B in Michlet, although the thinner the better.

A lot depends on the size of the transom, and whether it is running dry or
partly wet. Also, I would have to see your input file to judge whether you
are actually using Michlet correctly.

Yes, there are better tools out there but I haven't released my most recent
"Flotilla" set of programs which includes monos, cats, SES and ACV and
works very well for quite beamy hulls with large transoms. You will have to
wait for me to finish writing the manuals while fending off the Mongolians in
Civilization V.
I guess you could pay for someone to do the calculations for you, but that
might be well beyond your budget.

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DeeringSenior Member

That could work for me. I was hoping to reduce hp from 370 to 250, and cut enough engine weight in the process to counter some of the added hull weight.

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