Short Wave Sportfishing wrote in message
...
On 21 Nov 2004 17:35:54 -0800, (Big Daddy) wrote:
Hello. I have a 29' Chaparral (great boat) with twin v-6's and Volvo
duoprops. It comes on plane extremely fast if I work the trim tabs
full and use the outdrive tilt. You almost don't even know the bow is
raising!
My question is...in lieu of the damn gas prices, I'm wondering if I
could get better mileage if I added larger trim tabs so that the boat
would plane at a slower speed. Right now, I have to get it to 3,000
rpm's to bring it down on plane.
Any thoughts?
Based on my experience, I've never used trim tabs to bring a boat on
or off plane. On my Contender, it's more a question of lateral
balance in unfavorable sea conditions and for keeping proper bow entry
in seas. Then again, I have outboards, so the conditions may be
different.
I don't think larger trim tabs are going to help much because of drag
issues. I'm not all that familiar with I/Os, but with outboards,
plane is more a function of proper engine trim and throttle setting.
On my Ranger, a 20 footer which doesn't have trim tabs, it's all
about engine angle and throttle setting. On the Contender, I hardly
twitch the tabs unless I absolutely have to.
I would think that larger trim tabs would create more drag, thus less
performance, thus more gas used.
Later,
Tom
IMO and personal experience, the results of using trim tabs and probably
their optimum size varies from boat to boat, so there is no generic answer.
Like Tom pointed out, trim tabs are primarily for lateral positioning due to
heeling into the wind or unbalanced lateral loads. Many boats that are
under or marginally powered benefit from lowering the tabs to help get on
plane, but it is a balance of lifting the stern and generating more drag.
If the OP's boat is underpowered, my guess would be that larger tabs might
help lift the stern to get on plane which will the significantly reduce
overall hull drag. Once on plane, he should slowly raise the tabs back up,
while watching his tach until he finds the "sweet spot" of max RPM for a
fixed throttle setting.
Somewhere I think I remember reading that tabs should have an inch of width
for every foot of boat length.
Just my opinion.
Eisboch