Trim Tabs

[diver429]

Im thinking about getting trim tabs for my 1984 Seacraft sceptre hardtop with a 1999 200 mercury optimax on a armstrong bracket, can anyone make a suggestion on which trim tabs I should get for this boat? Also does anyone know what size prop I should be running for this boat. I have a 14 3/8 x 15 prop on there now and dont seem to be getting the rpms that I want. I m now getting 5400 wot at 35 mile an hour. And at cruise I get 25 miles an hour at 4600 rpms.I here guys getting the low 40s with the same engine on a walkaround. I cant see the hardtop slowing me down 7-8 miles an hour, I think its the wrong prop. Any ideas will be appreciated. Thanks Peter.

[McGillicuddy]

Peter,

Some guys would say your at the low end for power on a 23' Sceptre. I won't say that but I don't drive very fast.

Properly trimmed tabs will always help that boat achieve its great ride, but I don't think it will solve your rpm problem unless your transom is just riding way too deep and is pushing water.

Have you checked the motor height. I think the Seacraft standard seems to require the anti-cavitation plate being 1 to 1.5" above the keel at parallel trim. I think that changes like 1" per foot of set back on your bracket. Don't quote me on that.
Use a drywall square or something similar to measure that first. If the motor is to low it adds substantial drag and introduces less than ideal water/air mix around your prop, further reducing its efficiency.

Here are a few things to re-consider.

Your hardtop is like a parachute. Whether strapped on your back on the back of a dragster, it inherently creates drag and slows you down.

Your Sceptre windshield has twice the surface area of a walk-around. Consider it a second parachute. I know you don't believe it but those 2 things will easily slow you down 3-5 mph and will make your motor work harder. Take it off this spring and you'll prove it to yourself.

I'm not sure who's walk-around you've referred to, but Gary's walk-around has a brand new motor w/an extra 25 hp. And he did a lot of experimenting to get the motor height right & to get the performance he is now getting.

Your rpms at WOT do sound about little low, as I think your target is the 5500-6000 range. Raising the motor a hole or 2 could resolve that. Or not.

Regarding the prop, what make and model is it? Aluminum or stainless? Its size may be fine. Its shape (the model) may not be. The shape of the prop has a big impact on the motor's ability to turn it, some turn easier than others. A prop that has a lot of cup is harder to turn than a straight blade. Some give more stern lift some more bow lift. You might want to begin with that. What model is the prop you have and what is it designed to do? Are you riding bow high or plowing water?

[diver429]

Well McGillicuddy thanks for your reply. I spoke to a tech at armstrong brackets and he said for every foot that your motor is away from the boat you have to go up 1 inch. So you were accurate in what you are saying. My motor is 38 inches away from the boat so I already know I have to go up 1 hole already. I put a straight edge on the keel and leveled the motor with that straight edge and I measured 2 1/4 inches from the straight edge to the cavitation plate. So every hole is 3/4 inches, so if I go up 1 more hole I will be 3 inches above the keel which according to the tech from armstrong is where I want to be. Im afraid if I go 2 holes it may be too much. And yes I was referring to Garys boat and I read what was written on that post. My motor according to the manual should run between 5000 rpms - 5750 rpms, but I would prefer to run closer to that higher number as possible with 3 men on the boat and at least a half a tank of gas.I remember Fr.Frank sying that on one of the other post.I think you are accurate in saying that the windshield and the hardtop will add weight and drag. I also have a full canvas enclosure which is more of a parachute. But Im hoping that raising the motor 1 more hole will increase my rpms and speed. The prop is a Apollo michigan stainless wheel xhs 14 3/8 x 15 . I was reading on that post with Gary about a stern lifting prop, I wonder if that would help me in my situation. Well one thing for sure is I have to raise the engine at least one hole and then take it from there, because as it is right now I have water shooting up from the motor in a vee fashion at cruising speed.My problem now is I have to wait until the spring because its a little cold here in N.Y. right now and the boat is on land. I dont know if this is the right prop for the boat but I would definitely appreciate some feedback. Thanks guys.

[McGillicuddy]

The Apollo has a lot of cup and progressive pitch along with a lot of blade area, likely making it a harder prop to turn. It is also a bow lifting design. I really don't know what the preference is on a sceptre. You might see if your local prop shop will loan you a Mirage Plus or a Powertech MQS for a day in the same size. Maybe even try a Black Max in aluminum to see if the sternlifter is what you need. If that works look into PowerTech MQF series to get the same blade geometry in stainless.

Regarding the Armstrong rep, he's familiar with standard mounts. By most accounts Seacraft hulls want to be just a bit higher than normal. But 1 hole at a time makes the most sense.

Good luck. and Happy New Year!

[diver429]

Thanks for the feedback Mc Gillicuddy. You and everyone one else on this site has been very helpful. I wish I can leave my boat in the water all year like you guys in california and florida. Happy holidays to all.

[Blue197320]

The Apollo has a lot of cup and progressive pitch along with a lot of blade area, likely making it a harder prop to turn. It is also a bow lifting design. I really don't know what the preference is on a sceptre. You might see if your local prop shop will loan you a Mirage Plus or a Powertech MQS for a day in the same size. Maybe even try a Black Max in aluminum to see if the sternlifter is what you need. If that works look into PowerTech MQF series to get the same blade geometry in stainless.

Regarding the Armstrong rep, he's familiar with standard mounts. By most accounts Seacraft hulls want to be just a bit higher than normal. But 1 hole at a time makes the most sense.

Good luck. and Happy New Year!

sorry to thread jack but this might be useful... so the aluminum blackmax is a stern lifter?!!?!

[McGillicuddy]

That's my understanding per Powertech Props. They have duplicated the geometry of the OEM Black Max for their MQF Stainless Series.

http://www.ptprop.com/images/stories/docs/dclass.pdf

I just bought a Black Max to give it a go. If its the right prop I'll ante up for the stainless version from PT.

I just read some interesting tests re value of Stainless vs Aluminum and it was eye-opening for offshore use. They showed increases in fuel efficiency, speed and thus range.

[Fr. Frank]

... so the aluminum blackmax is a stern lifter?!!?!

Yes. Designed to lift stern-heavy sterndrives. The design predates V6 outboards, but still is unmatched for an all-around sterndrive prop. The new Alpha 4 comes close, though.