Crouch number

Power Boats only. Please include the boat type in your question.
dennisb
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Crouch number

Post by dennisb »

I am looking at building a DE25 and would like to get some idea of performance with different motors (HP).

I have looked through a number of posts covering crouch numbers and they range between 150 and 190. the most accurate appears to be 155 to 165 (using the data supplied in the posts).

My question is, for the DE25 is crouch number ~ 160 reasonable ?

Thanks for the input



TomW1
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Re: Crouch number

Post by TomW1 »

When I have propped the DE/NV series boats I have used 190 as the C factor. I no longer use Crouch as I have a total formula that calculates speed and recommended propeller diameter and pitch. Your 155-165 is used for deep V hulls which the DE25 is definitely not. Speed is also very dependent on weight. The study plans give you 2 speeds with a 150HP at two different weights. Until you have the boat built and weigh it Crouch numbers are really meaningless. Weight is the most critical factor in performance, every 200lbs changes the prop pitch needed and your speed.

Please feel free to ask me any questions you may have, I have propped many boats for the members of the forum.

Tom
Restored Mirror Dinghy, Bought OD18 built by CL, Westlawn School of Yacht Design courses. LT US Navy 1970-1978

jonnymac
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Re: Crouch number

Post by jonnymac »

Matt Gent has a detailed performance report on his de25, I’d use some of his real world numbers as a sanity check. Make a table of different hp and weight combos based on what you think you will end up with whether you go with a de25 or a de25+10%, and compare it with his numbers and you should be able to get a expected range.

dennisb
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Re: Crouch number

Post by dennisb »

Thanks for the input.

At this point I think I need to build the boat and weigh it. From there I can determine what power I put on it. I know the performance I want, max 30 MPH, cruse 20 - 22 MPH.

TomW1
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Re: Crouch number

Post by TomW1 »

Dennis once you have it built get in touch with me and I will run it through my formula's and come up with motor size and prop size. Also make sure you don't miss anything in your weight calculation. Just got done finishing two members calculations.

Tom
Restored Mirror Dinghy, Bought OD18 built by CL, Westlawn School of Yacht Design courses. LT US Navy 1970-1978

Matt Gent
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Re: Crouch number

Post by Matt Gent »

Unless you’re planning something drastically different you don’t need to build it to pick power. I’ve posted lots of data, including measured weights. As best I can tell, except the bracket, my boat is built to plans, with a fairly basic interior / outfitting. Crouch comes out 185-190 using the engine’s labeled hp.

Depending on what I’m doing, loaded weight can vary by over 1000lb, much more than any choices in construction.

The boat won’t want to run much over 30 except on the calmest of water. Trim tabs help a lot with comfort at speed. Suzuki 115 or 140 are a good match, as well as the Merc 150. Hard to beat the 140 for power/weight/ cost. I think with the 150 cruise rpm would be lower, maybe a little quieter.

If you are ever in south FL shoot me a message and we can go for a ride.

dennisb
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Re: Crouch number

Post by dennisb »

Thanks for the input.

Presently looking at plywood pricing (ouch)

TomW1
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Re: Crouch number

Post by TomW1 »

dennisb wrote: Thu Oct 21, 2021 8:31 pm Thanks for the input.

Presently looking at plywood pricing (ouch)
Dennis get a quote from here they have great prices and even with shipping you may be ahead.

Tom
Restored Mirror Dinghy, Bought OD18 built by CL, Westlawn School of Yacht Design courses. LT US Navy 1970-1978

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