Triple-d wrote:Hi Joe
I have a 27' True Grit hull with a 170 hp cummins mercruiser diesel with a bravo 2 leg. Hull weighs 8500lbs with a 67 gal fuel tank. My top speed is 19.5knots, 9.8gph. I cruise at 2800rpm which is 61%eng load, 12.7knots and 4gph. Idle is 3.4 knots and .2gph. Too fast for our fishing trolling so had to put a 15hp kicker motor on. Hope this helps and good luck with your build. Don
Thanks for the info! Glad to hear another successful True-Grit builder is around and with all the builds i've been looking at I saw your pics on the Power Requirements thread but didn't remember that it was a True Grit design. My apologies, as she's a beauty. Your swim platform and transom door are good ideas and executed nicely.
With rough calculations figuring a 10% loss(not sure what i/o's lose in efficiency) at 61% engine load you're putting out about 93.3hp to get 12.7kts and 4gph.
Sorry, but I'm not agreeing with those numbers.
you should figure a 16" dia prop, and personnaly, I would squeeze in a 17" (leaves you the 4-blade option)
I am running the 3.0L with 2:1 gear
I have hit 30mph on a good day, you should easily hit 20.
When you stretch that design, it is a semi-displacement.
If you used displacement hull numbers, it wont work.
Use semi-displacement formula.
I have contacted some prop shops that were touted a 'Gurus', and they told me my prop was cavitating, all sorts of reasons my boat wont go.
I got another prop shop and all is well.....
Kens, I welcome your disagreement as it's helping me understand everything much better, may not seem like i'm getting it though
I wasn't too sure of those numbers but figured it was a place to start. That online calculator didn't have the ability to change the displacement hull numbers to semi-displacement.
Also, while our hulls are similar, I believe they are different in a specific way related to planing. If I recall from my reading on these forums, this was discussed before. I thought your aft deadrise on the double eagle was at 1* where the TrueGrit is closer to 5*. I might be wrong but I thought someone described the difference as being the double eagle was a semi-planing/near-planing hull and the True Grit was a semi-displacement hull. The result being that it takes less power to get on plane than the True Grit. I could very well be wrong as i've read so many threads in such a short time.