Cat power and prop calcs

[dboatsafrica]

Malcolm Tennant was a great advocate of long and narrow displacement hulls and I have followed his approach with an 11 metre cat designed for operations on the world's second largest lake. Hull slenderness ratio of around 13:1, max displacement of 2000kg.

I have been using square root of WL x hp/displacement to calculate speed and this indicates that 2 x 60 hp outboards should provide a cruising speed of around 23 knots which is all I require. (Is there a better formula ?)

I am planning to use Mercury 4 stroke 60hp Bigfoots but mercury's prop size calculator says it does not have a suitable prop. I did download a prop calculator which indicated that a 14" Diameter prop with 13" pitch would be ok but the calculator is only for monohulls.

Malcolm is sadly not around to ask any more so I'll appreciate any advice members have to offer about my power assumptions and getting the right prop. Thanks.