Boat Trim

Post your technical questions or solutions about your boat's hull, prop or rudder here.

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TBOT422
Posts: 353
Joined: Sun Jun 26, 2011 5:36 pm
Location: Clearwater, FL

Boat Trim

Post by TBOT422 » Sat Sep 28, 2019 8:46 am

How much does your stern drop when under power? I constantly search for ways to improve boat speed (maybe that is because I am an ex-power boat owner), but we don't ever seem to be able to achieve the speed that most people on this forum talk about whether under power or under sail. I have began to notice that our stern lowers into the water significantly when under power. Given the squared off transom of the C350, that can't possibly do anything but increase drag and lower speed. At 2400 RPM we typically can achieve 5 to 5.5 knots with a recently cleaned bottom whereas the typical discussion on this forum is around 6. The only time we get 6 knots or more is when we have a helping tide or breeze. I have converted the aft berth into the 'aft garage' and there are a lot of tools, spares, toys (inflatable kayak, roll-up dinghy, etc.) stored there, but others have dinghies stored on davits even further aft than what we have. We generally keep the aft tank empty to reduce weight aft. I wonder if I need to move some of that stuff forward under the forward berth. Or possibly it is simply a matter of the keel wing or prop shaft angle that creates the effect. We have the standard "M" series prop (newer wide blade). Our stern drops almost 4 inches when under power, what is 'normal' for a C350?
Gary & Janet
The Best of Times (Hull #422)

wolfe10
Posts: 365
Joined: Mon Dec 11, 2006 2:58 pm
Location: League City, Texas

Re: Boat Trim

Post by wolfe10 » Sat Sep 28, 2019 9:01 am

I know this is not a direct answer to your question, but with your low boat speed, my first question has to be-- wonder if you are under-proped?

Correct prop is not that difficult to determine.

Go to WOT-- wide open throttle.

If RPM instantly goes to governed RPM, you need more prop (diameter and/or pitch).

If you are 400-500 RPM below governed RPM (and likely black smoke in exhaust), you need less prop.

Ideal is with a clean bottom WOT results in 150-200 RPM below governed RPM.
Brett Wolfe
C350 #180
"Vindaloo"
League City, TX

User avatar
TBOT422
Posts: 353
Joined: Sun Jun 26, 2011 5:36 pm
Location: Clearwater, FL

Re: Boat Trim

Post by TBOT422 » Sun Oct 06, 2019 11:54 am

I haven't had the time to check the WOT tests provided by Brett, but I did happen to observe the 'trim' one day while sailing. The stern does not sink when under sail, so the drop is clearly caused by the angle of the prop shaft and is not due to load distribution or keel wing 'lift' or any other things that I can think of. If it is simply created by the fact the prop is 'pushing' the bow upward, there is nothing that can be done about it. It is what it is.
Gary & Janet
The Best of Times (Hull #422)

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