15hp to 30hp

[georgieboy]

hi, i have a 3.5m rib with a tohatsu 15hp four stroke. it currently does about 22mph wot.
the transom is rated at 20kw so i assume that a 30hp would be the max i could put on it. my question is that if i put a 30 on the back instead of the 15 how much extra power / speed would i get?

[Poly]

the transom is rated at 20kw so i assume that a 30hp would be the max i could put on it.

there's some optimistic rounding in there - and 25HP might be the "safer" approach!

my question is that if i put a 30 on the back instead of the 15 how much extra power / speed would i get? [/quote]

Its difficult to predict these things accurately but the "theory" goes something like this:

(Speed)^2 = (HP / weight) x (hull constant)

The constant depends on the hull type etc. Since its the same boat we are talking about here then the hull constant shouldn't change. So ignoring the fact that a bigger engine will be heavier - doubling the power will make it about 1.4x faster. i.e. 22 mph becomes 31 mph. Slightly less because your going to add something like 10% to the total weight.

[NSS]

Not sure how much help this will be, but I run a Tohatsu 18hp 4-stroke on my Avon 3.4m. I'd have liked to put a 25hp 4-stroke on it but, although it would have been within power limit, it would have been over the weight limit for the transom. You might want to check that you wouldn't suffer the same issue.

The Tohatsu 15/18/20 hp 4-strokes are the same block and weight but the 25/30 hp are bigger and significantly heavier.

By the way, my 18hp gives a max of about 25mph.