What length and weight must my boat be to achieve 50km/h with a 39-HP outboard Diesel?

I am currently building my own boat. My plan is to find an aluminium hull, attach a 39-HP Yanmar outboard Diesel to it (available in Australia, in case you wonder if I can't spell "Yamaha" properly or got my facts wrong because yes, it really is a diesel, too), and put my own superstructure on it. It's supposed to be a commuter boat, a glider, with which I hope to cover 12km in an inlet every morning and night. It's salt water, usually calm as a lake, but can have a wind-induced chop of up to 1m at times.


I'm grappling with the question now whether I should get the smallest possible boat (3.3m) or go a bit bigger to get better planing ability (up to 6m is what I was thinking).


Thanks for your help!

What length and weight must my boat be to achieve 50km/h with a 39-HP outboard Diesel?
The longer the waterline the faster the boat will go. Also I would go with the larger size for the times when there is a 1 meter chop.
Reply:I'd love to have a b/f with a boat or even just building one. What a great question too.
Reply:not sure, but i think you better consider 5m. instead of 6 and add a little to the bottom width and beam if you want the speed you mentioned.
Reply:scrap the diesel and get a outboard, 39hp yanmar would be lucky to push a 6m alloy boat at 18kts
Reply:to answer specifically, would be hard, as there is no set weight or length per speed, its not proportional. factors such as prop, width, hull form etc all play a part.


however, from what you say, id be looking for a shallow V hull. this would give you the best speed and performance. you sacrifice some comfort if you do encounter 1m swell, as shallow Vs tend to slam a bit more than a deep V, but, less wetted surface area (less boat in the water) more speed. and shalow v more suited for lakes or calm water.


weight, if your only going for a 39hp, would need to be kept to a minimum. sounds like you already have your heart set ont he yanmar, but personally, id stay away from the outboard diesels, they never have really been much good as an outboard. great inboard engines, but lack the power for as outboard. also they are generally heavier than similar powered 2 or 4 stroke engines. and on a boat the size your talking about, engine weight is omething you want to consider. if you not headset on the diesel, might be worth looking into a 35 - 40 hp outboard 2 stroke or 4 stroke, 2 stroke has better performance, 4 stroke better economy. but a diesel will work.


5 meter or so would be ideal id say. however, you might struggle to meet your speed wants with an engine that small. would be pushing it hard to keep that speed. planning ability, is down to hull form, not length really. i had a 4 meter flatacraft, that was shallow V and had a 35 johnson on it. it would get up and go no problems, but, load it up too much, engine struggled to get it up on the plane quickly. once up it can maintain it, its getting it up thats often the problem (hold the viagra comments)


so shallow v, 5 meter, and your choice on engine. keep the weight down to a minimum. if your going to build the rest yourself, avoid heavy woods or metals. be wise to look into fiberglass, if your not familiar with fiberglass, could be worth looking into it. much lighter and just as strong. wood can mount up to some heavy weights quickly, and metal, being salt water, your limited to either marine grade stainless steel, which would cost a fortune and weigh a ton, or marine grade aluminium, which much lighter, but cost a fortune.
Reply:are you going uphil or downhill?