how much of my Alpha 1 drive should I see under full power? Can't get proper speed.

[JackBronson]

OK, here goes: 1987 Starcraft 19' bow rider with Mercruiser 5.0 230hp and Alpha 1. Tonight out in the harbor we have the throttle set so the motor is running about 2800 RPM and the best we can manage is about 23mph, when I figure we should have been around 30mph (based on past experience). About 800 pounds of people and gear on board at the time, spread out on the boat. for the heck of it I look over the transom and I can see the outdrive lift cylinders no problem as they are not covered with water. after measuring it, this means from the side of the cylinders to the top of the prop that's about 5 inches of water, which doesn't seem like enough.

I have the SX trim tabs set on the 3 hole. I had the trim on the boat proper (moved it up and down as needed) and the boat was actually set up flat as if it wanted to plane out. It was somewhat planed out (credit the trim tabs I believe with the low speed plane capability) it just didn't have the speed it should have, and giving it more throttle just revved the engine more and I was approaching redline so I wasn't risking any more.

At lower throttle, putting up the river, the thing is fine. the drive is way down in the water, fully submerged at low speed. but this is the second time I've had 5-6 people on board in the past two weeks and it seemed to be topping out with not much power. Last night we had three on board and it seemed pretty good, but I wasn't paying attention to it because it did not become a confirmed problem until tonight.

I'm running the same prop this year as I ran last year, and last year after putting on the trim tabs I hit 44 on the lake with two people on board.

if it was a car I'd say the tranny is slipping under heavier load. Not sure how that translates to a boat. Anyone have any ideas?
Thanks in advance.

 

[Bt Doctur]

Re: how much of my Alpha 1 drive should I see under full power? Can't get proper spe

tabs should be retracted all the way, drive should be "down" all the way.once on plane adjust tabs for level operation and trim up for max rpm, adj throttle accordingly.with sufficiant horsepower you should see the cavitation plate on the surface behind the transom.

 

[alldodge]

Re: how much of my Alpha 1 drive should I see under full power? Can't get proper spe

The SX type trim tabs use an internal spring and will allways produce a downward force. You said they are set in hole number 3 which is the mid range setting, for better top end bring it to hole 2 and then 1. These holes provide reduced downward force so when on plane you will have less drag, this will also reduce the time of which you get on plane. You are getting the low speed planning capability you wanted but your loosing because of drag. The tabs are putting a constant drag on you boat and even when placed in hole one will be slowing you down a little. I would not recommend none adjustable trim tabs for performance. It would be nice if a stringray type fin could be put on your boat without drilling holes, but it cannot. Have one on my 23 footer and it does very well, I loose about 1 to 2 mph at top end but everything else I'm very pleased with.

 

[Don S]

Re: how much of my Alpha 1 drive should I see under full power? Can't get proper spe

There is nothing in an Alpha drive that will slip and continue giving you high rpm operation.
Do you set your trim level by the gauge or by feel? I hope it's not by the gauge alone. and looking to see how much drive is out of the water is not a way of checking trim either.
Since you also say you were nearing "Red line" (no such thing on a boat) and still going slower than normal, I would say your tachometer may be off. Maybe reading too high.
You also need to check your wide open throttle rpm. Should be 3800-4200 rpm assuming you have a 5.0L, and not the 5.0LX.

 

[JackBronson]

Re: how much of my Alpha 1 drive should I see under full power? Can't get proper spe

Thank you for your replies.
Bt Doctur - based on what you said it sounds like the amount of the drive I am seeing (lift cylinders, top of anti-cavitation plate) is normal when on plane. that's good news.

AllDodge, are you saying that if I drop down to the #2 hole or the #1 hole on the trim tabs I will increase my top speed as I will be decreasing drag? This sounds correct. I am going to be on a lake tomorrow so I will try out the performance, although I think I will lose the thing I like the best about the tabs, which is the low speed planing. I got these because I have major bow lift when taking off, and these trim tabs resolved that.

With all that said, I would still think that I should be able to get this boat going faster even with those in the #3 hole. I still think I should be able to get to 30mph at full throttle with a Merc 230 in a 19' bow rider with 5 persons on board.

 

[JackBronson]

Re: how much of my Alpha 1 drive should I see under full power? Can't get proper spe

There is nothing in an Alpha drive that will slip and continue giving you high rpm operation.
Do you set your trim level by the gauge or by feel? I hope it's not by the gauge alone. and looking to see how much drive is out of the water is not a way of checking trim either.
Since you also say you were nearing "Red line" (no such thing on a boat) and still going slower than normal, I would say your tachometer may be off. Maybe reading too high.
You also need to check your wide open throttle rpm. Should be 3800-4200 rpm assuming you have a 5.0L, and not the 5.0LX.

Don, thank you for your reply. My "redline" comment was made because I observed that for the MCM 230, Mercruiser specifies the Max. RPM (at WOT) to be 4200-4600 - the mistake I made was committing to memory the spec for the MCM 200 (3800-4200 RPM) which is listed in the same table in service manual #9. the good news for me is I just picked up an additional 400 RPM, as I make sure that the throttle is not advanced to such a point that the motor will be above the Max RPM spec.

I definitely set the trim by feel. I can tell when it needs to go up or down, and must say that learning this and seeing the difference has been one of the more satisfying aspects of operating my first boat. I only looked over the transom because it seemed something wasn't right (and my son was driving while I was thinking through the potential causes).

I have an external tach that I will hook up and test against the boat's. Checking the tach is a good suggestion.

Perhaps I am imagining that there is a greater issue, when in reality adjusting the trim tabs to reduce the drag and hitting the throttle harder beyond my artificially low mental preset of 3500 RPM (which means I stay low 3000's) is the problem.:embarassed:

 

[Don S]

Re: how much of my Alpha 1 drive should I see under full power? Can't get proper spe

the good news for me is I just picked up an additional 400 RPM, as I make sure that the throttle is not advanced to such a point that the motor will be above the Max RPM spec.

You don't understand. The way boats work is you should be able to run wide open throttle and the engine not go over the WOT spec for your model engine. If it does, you change prop pitch to get into the correct range for your engine. Then you are propped correctly and can run at any rpm you want

 

[JackBronson]

Re: how much of my Alpha 1 drive should I see under full power? Can't get proper spe

OK. So to ensure I do understand, this is what is supposed to happen: with the correct pitch prop, I can throttle all the way forward and the motor should build to but not exceed the WOT spec. IF it does, then I need to change the prop to a pitch that allows for WOT operation without exceeding the spec.

Thanks for the clarification, Don.

 

[Don S]

Re: how much of my Alpha 1 drive should I see under full power? Can't get proper spe

OK. So to ensure I do understand, this is what is supposed to happen: with the correct pitch prop, I can throttle all the way forward and the motor should build to but not exceed the WOT spec. IF it does, then I need to change the prop to a pitch that allows for WOT operation without exceeding the spec.

Thanks for the clarification, Don.

Yes,

What you need to do first, is confirm your tach is correct, then run the boat with normal load and find out what your actual WOT rpm is. That is the number you will use to calculate what the prop should be changed to.

We also have a prop forum to help you find the right prop.