Should I add Power Trim?

[roger3645]

I’m running a 1977 tri hull bass boat with a 1975 85 hp mercury that does not have power trim ( with 135 psi in all 4 cylinders). With the manual trim I have the trim pin in the second hole from the bottom. The dry weight of the boat is 850 and I’m running at 5300 rpms WOT, with an aluminum 3 blade 13 x 21 pitch prop, which pushes the boat at 34 mph by gps. On ebay, I see the old style power trims (dual ram systems) for about $150.00. I’ve been thinking of adding one of these units. But first I wanted to ask what type of performance increase to expect if any. I’m looking for an additional 5 mph at WOT. Will I get it? Also, if I move the manual trim pin up just one hole to the third from the bottom the boat proposes. Am I beating a dead horse here or is there a way to get a bit more from this old tri-hull, other than strapping on a larger motor. It’s rated for 90 hp. Thanks

 

[LubeDude]

Re: Should I add Power Trim?

I have had two tri hull early Bass boats, one, a 15' Ranger with an 85 HP evenrude, and a 15' Skeeter with a 75 evenrude. Both had power trim and both would run at 45MPH or just a bit better on a good day.

You will not be sorry you did it and it will be worth it. You will have to play with the hieght of the engine and try different props but 45 MPH is reachable.

Now, if you were to sell the 85 and purchase a nice clean 90HP with power trim, I would think that 50 wouldn`t be far off. That would be my choice if I had the money.

 

[steelespike]

Re: Should I add Power Trim?

No doubt power trim is the nuts.But in the mean time
you could try raising the motor vertically.You could also make some wedges to bring the motor in 1/2 a notch so when you go to the third notch maybe it won't bounce.
You won't get 45 but you could get 2 or 3 mph if you can gain an inch in height.

 

[tashasdaddy]

Re: Should I add Power Trim?

if you really like the boat, and motor is is good condition, go for it. once having PTT you'll never go with out it. even if you don't get the speed you want, it becomes a necessary convenience.

 

[Texasmark]

Re: Should I add Power Trim?

Seldom is one able to have the best running angle on the engine with manual tilt.

Reason being is that the best WOT engine attitude would cause problems at the lower speeds...like porposing or horrible hole shot.

If you are able to run your boat without the above problems, it says that you are in for some more mph, maybe the 5 you are looking for by installing trim.

If you answered yes to the previous paragraph you have told me (by definition) that you are "plowing" at WOT and when you lift the bow outta da watta you'll get your speed...............but not with an alum prop!

If you are going to get some serious speed out of that rig you need a serious prop.....in particular, a prop with thin blades among other things like cupping and high rake and all that tech stuff. BTDT....and I used to run an early 80's 85 4 banger with PTT.....but 39 was the best I could do.....but I could have spent some money on props and done better. At the time, was lucky to have a boat at all. d ......and, thanks to Kenny and iboats, I know a lot more about them now than I did then.

I just ran over to the prop slip calculator and plugged in your numbers and it said that at WOT your prop slip is somewhere between 26 and 36%.........horrible.

This says 2 things; your prop is letting you down and you are plowing which is putting extra pressure on the prop which adds to the slippage. I don't remember the gear ratio for that engine but I used 2.3:1 (26%) and 2:1 (36%). The lower the ratio the worse it gets.

So, this supports what I told you.

I put in some hypothetical numbers. Said you had a 2.3 ratio (lower ratio would yield better numbers). Said that you would pick up 300 rpms by getting the bow up which significantly reduces hull wetted area and hence drag....not hard to do.....now WOT is 5600 rpms.

Stayed with 21p but this piece of data is based upon (your getting) a high performance SS prop.....this allowed me to gustimate the slip down where it belongs at say, 12%.

That put you at 42 mph. That what you're looking for?

My 2c,

Mark

 

[roger3645]

Re: Should I add Power Trim?

Thanks guys for the great response! Got some great info from you guys. Okay I’ll keep an eye out for a power trim unit. I’d really like to strap on a 1984 or later 90 hp, that would be equal to 115 hp in 1977 (yr of boat) ratings. I have another 1975 85 hp mercury with over 150 psi in 3 cylinders and 90 in #3. I plan on making it my winter project. Maybe with the successful sale of both 85 hp motors I can get a 90 with power trim. But that Honey DO list is fairly long. Well thanks again guys! And the first thing I’ll try is raising the height of the motor. I really didn't know that an SS prop could make such a dif.

 

[PuddleJumper]

Re: Should I add Power Trim?

You might look into universal power trim unit. I think iboats has them. Would give you power trim with new hydraulics and function a little like a jack plate as well
FWIW

 

[roger3645]

Re: Should I add Power Trim?

Well just to let you guys know I just purchased a complete power trim system off ebay. Hopefully it'll get here before Veterans Day so I can install it. Thanks Again for all the advice.

 

[Texasmark]

Re: Should I add Power Trim?

Good luck.

Mark