Displacement powercat single 40hp

Cheers mate

I wouldnt know how heavy either, I dont have the means to even make a guess. How light do you reckon a rig like that can be built using ply/glass hulls and aluminum deckwork? basic ply/glass cuddy with double mattress and toilet/shower as well as outdoor bbq?

 

Draw something up. Take a photo of it then post it here. Then we can make estimates. ..

 

You could always plonk the single engine at the point of pitch or thereabouts, but how you steer the thing, then, becomes the problem.

 

Groper, is the camera boat some sort of secret project? Been searching online for some details on the boat or videos, nothing apart from some vague pics come up. Do you have any links to that or like boats?

 

Trying to locate this vid again, please help

 

Found it

 

This is awesome, takes twin 40s but look at her go towards the end of the vid... 25 knots!

I need to make myself something like this

https://www.youtube.com/watch?v=grDqbKohm1g

 

We have built a displacement catamaran that is 24' with twin 20 hp Yamaha 4 strokes that goes 21 knots. It is very nice in 2' chop especially if driven above 16 knots. Lower than that it it will start to pitch. We added hydrofoils to lift 60% of the displacement.... Curved main foils and custom foils with trim tabs on the outboard. Now we can go 24 knots and the best part is the ride is glassy smooth in chop. The foils help pitch but also roll dampening so the boat feels like a lot bigger boat... Equivalent to a 28' deep v cabin cruiser in ride. On the center mounted 40 option- I really like twin outboards for maneuverability and safety. But... The Yamaha 20's are very low tech motors and are not so fuel efficient. I am getting about 10 hp per gal per hour burn. That is what a two stroke gets. A single 40 hp with electric fuel injection would do a lot better... Probably 15 hp per gal per hour. Now mercury/ tahatsu have 25 hp efi motors with power trim tilt. I would try those. Another thing is weight issues on a small narrow hulled cat. To make it work the boat needs to be light and the hulls need to be skinny. The cat we built has a length to beam ratio of 17:1 for the hulls and is the reason we can see these speeds in displacement mode. 14:1 would be the minimum for a high speed displacement cat.

Hope some of that helps you.

 

Nice.... do you have any pics showing the foils? I've been thinking about the same configuration on my almost completed 35ft cat powered by twin 90hp suzukis. ..

 

Thanks for the input mate, some good info there. I think I will aim for testing out a single 40 first and if not happy with the performance then run twin 40s. I really find what you have done with the foils interesting. I was rambling on about some foil assisted lift earlier in this thread and good to see some input that might point me in a direction to play around with

Groper posted a link to a primer on optimising cat dimensions in a thread elsewhere on boat design, I came across that and found it to be a big help in starting to understand the cat hull design process. I am currently playing with Hulls from Carlson design to draw some lines but finding it tough going as it only caters for hard chines and lots of chines to more rounded

 

As a side question, I want to learn about the effects of having a flat bottom on the slender cat hulls? How badly does it affect ride quality in chop vs any assist in lift giving a touch of planing surface?

 

I have a thought which may be wrong but can you build your cat as normal with twin wells or pods and just mount the 40 on the opposite hull to helm position. that way you don't have to build a central nacelle and if you can a afford a second motor later it wiil just be a bolt on fit. the small markham whaler cats used this idea for years.

 

most cats I have seen have fairly flat aft sections. I would imagine the ride should not be to bad as the hulls are very narrow anyway.

 

Interesting, I could try that

I also been having a think along another line, has anyone come across any torpedo types nacelle being used in front of a single motor on a cat?

 

17:1 is extremely slim for a small boat, the hobby-horsing is how bad ?