Can trim tabs slow boat down?

[FillupD]

I have a 1973 50 hp evinrude and just installed a teleflex tach and it reads 4000 WOT.
I also just installed smart trim tabs on a 16' Lund. Its like a Mr. pike but it is a 1978. I think my tabs were may be too stiff. 60# I think?
Can the trim tabs restrict enough to reduce RPM's?
That motor wouldn't have a slow mode would it. I didn't find anything in the manual.
Some info states to set pulse at 6 and some info states set at 5. I set at 6.

It has a 15" pitch prop. 27 MPH. I think I would have to go close to an 11" to get RPM's up.
Any opinions would be appreciated.

 

[Chris1956]

Re: Can trim tabs slow boat down?

Smart tabs (or any tabs) are designed to lift the stern and lower the bow. On most boats, max speed is obtained with max motor trim and zero tab. So any tab has the potential to lower top speed.

When I bought my speedboat, the PO had installed a heavy aluminum plate on the transom. It hung an inch or more below the bottom of the hull and acted like trim tabs. Top speed was 45MPH, with much plowing of water by the bow. I subsequently cut it down flush with the transom, and top speed increased to 60+ MPH.

 

[FillupD]

Re: Can trim tabs slow boat down?

Thanks, I'll take them off and try again. I was hoping to get my RPM's up but I talked to the local evinrude dealer here and he thought without power tilt and trim, 4000 to 4300 rpm is all I will see out of an older motor. It has very good compression though. I'm not sure if I could even prop it out enough to see the recommended WOT rpm. Then I would only be going about 10 mph.
I ordered my Ken Cook manual yesterday, I'll study it some and try to learn something that may help.

 

[cdn-pk]

Re: Can trim tabs slow boat down?

I had to calibrate my new telefles tach. made sure it read Zero with the ignition on. Then hooked up a know good tach and took the boat out and ran motor at 3000 rpm as stated on the good tach adjusted mine to read the same. then all was good.

on these deflection gauges it is most acurate between 10 and 2 o'clock. thats why it's calibrated in that range (i.e. 3000 rpm).

So i'd make sure the tach is actually giving you acurate info. before i calibrated mine it would read 4800 when the good tach read 5400.... pretty big diference.

cheers

 

[FillupD]

Re: Can trim tabs slow boat down?

The motor sounds like its about right at idle and the tach shows close to 1100 and close to 1000 when in gear. It may be a little high but it seems to work and start well at these settings. It just seems like there is a little more there when it is at WOT and it just can't quite get to it.

 

[jay_merrill]

Re: Can trim tabs slow boat down?

Sure. Trim tab create drag and drag will reduce speed. What you have to factor in, however, is how much they improve the attitude of the boat, during you normal cruise speeds. If they prevent the hull from mushing through the water, which would most likely create more drag than the tabs, you have a net gain.

 

[Chris1956]

Re: Can trim tabs slow boat down?

Also experiment the the tilt pin setting. Normal setting is second from lowest. Sometimes third from lowest works better.

 

[FillupD]

Re: Can trim tabs slow boat down?

I've played around and I am at 5100 at WOT with a 15" pitch. The tabs are at the weakest hole position and the boat handles like a sports car. I know I can get a little more top end RPM with a 13" prop but would my top end speed really go up very much if at all? I'm running 30 MPH now.

 

[FillupD]

Re: Can trim tabs slow boat down?

OK I finally got it up to recommended WOT. I even have to back off a little from 5900 RPM. My new hustler 13" prop dropped me down to 22 MPH but up on plane faster and snappier from dead start. I guess when I have a passenger and loaded I'll run the 13 and when I'm alone and want a little more speed I'll slap on the 15". I have a 19 and a 21SS pitch props if I really want to drop the RPM's down. Looks like I have some paperweights.