Gearbox ratio and prop size?

[scottydog78]

Hi,

I've recently purchased a used Seaswirl 175 which was in need of substantial repair. We've had the engine changed (Volvo Penta 3L I/O) though the boat won't get enough speed to plane. The engine seems fine so we're assuming it is either the gearbox or (less likely) the propellor pitch. Does anyone know what gearbox ratio this boat should have? And also the standard propellor size too? Currently ratio is 1.97 with 17" prop - might this be the problem?

Many thanks!

 

[Don S]

Re: Gearbox ratio and prop size?

That gear ratio is correct for the engine, and that prop should get you on plane. Do you trim the drive up as you start picking up speed? If you have the drive trimmed all the way down, it won't plane and you will just plow through the water.

 

[QC]

Re: Gearbox ratio and prop size?

You should probably post some sort of year model reference, but 1.97 is right for a 3.0. There are many reasons a boat won't plane which is kind of handled by this list: http://forums.iboats.com/showthread.php?t=295070

I would start with number 12. Particularly I would want to know about potential water logged foam. And do you know how to trim properly? Also, assuming the engine is right scares me. What was replaced? By whom? With what? In fact, assuming anything is where things get sideways fast. If this is not an engine or drive issue, then we may want to move this to the general boat section. But let's see how it plays out first.

BTW a 17 inch prop seems like a good place to start along with the 1.97, so I don't see that combo as a problem for now.

Welcome aboard! Good luck

Edit: Sorry Don, posting at the same time, but I got sidetracked.

 

[scottydog78]

Re: Gearbox ratio and prop size?

Thanks folks...it is a 2000 boat with a 2005 engine. Thanks for the list of issues - I've passed it through to the mechanic though he's still scratching his head to why it won't work - based in UK where there are very few Seaswirl boats so just checking in case the Seaswirl 175 has a particular set up in term of engine, gear ratios and props that we need to follow. Can't find a nailed down spec for a Seaswirl 175 anywhere!!

On specific issues, the trim works fine. Engine was replaced at the weekend by the mechanic - like for like switch of volvo penta 3.0l.

The engine is revving well in neutral, but when moving it won't go past about 3000 RPM - something is holding it back. After checking for waterlogging etc, his initial suspicions were that prop or gearbox may be non-standard or don't combine well together on the seaswirl. Other things he's checking out are things like the fuel pipes to see if there's any restriction there.

Bloody boats!!

Any help much appreciated!!

You should probably post some sort of year model reference, but 1.97 is right for a 3.0. There are many reasons a boat won't plane which is kind of handled by this list: http://forums.iboats.com/showthread.php?t=295070

I would start with number 12. Particularly I would want to know about potential water logged foam. And do you know how to trim properly? Also, assuming the engine is right scares me. What was replaced? By whom? With what? In fact, assuming anything is where things get sideways fast. If this is not an engine or drive issue, then we may want to move this to the general boat section. But let's see how it plays out first.

BTW a 17 inch prop seems like a good place to start along with the 1.97, so I don't see that combo as a problem for now.

Welcome aboard! Good luck

Edit: Sorry Don, posting at the same time, but I got sidetracked.

 

[Don S]

Re: Gearbox ratio and prop size?

Have you tried trimming up when it won't go any faster? That is the question, not if the trim works or not.

If you have the drive all the way down, it will plow through the water and your rpm's will not go up until you trim up.

 

[QC]

Re: Gearbox ratio and prop size?

Have your tech verify the ratio by turning her over in gear and counting prop to engine. We just had one of these yesterday where the gear ratio was changed and no one knew it.

Also, what was actually replaced? Short block, long block, complete engine?

 

[scottydog78]

Re: Gearbox ratio and prop size?

Short answer - I don't know. It has only been tested by the mechanic this morning. Is operating the trim not something obvious that he should know about? Excuse my newbie ignorance!

 

[scottydog78]

Re: Gearbox ratio and prop size?

Will do, cheers. Complete engine replacement. We originally had the long block replaced but there were issues with the carb and the rest of the engine wasn't in great condition - hence the replacement engine instead.

 

[Don S]

Re: Gearbox ratio and prop size?

Short answer - I don't know. It has only been tested by the mechanic this morning. Is operating the trim not something obvious that he should know about? Excuse my newbie ignorance!

Is he a marine tech with an established marine business?

 

[scottydog78]

Re: Gearbox ratio and prop size?

Yeah, he's been running his own business for a few years and worked for bayliner before that. He's confident that the set up should work in theory, though as there aren't many seaswirls over here he thought there might have been something peculiar about the seaswirl spec. I'd love it to be something simple like the trimming, though perhaps that's too much to hope for!

The consensus seems to be that the setup should work, so we'll have to suss out other options/possible causes. The 17" prop isn't fantastic (though would be surprised if it is the full source of the problem) - if we change it is there a better size we should got for with this set up or stick with 17"?

 

[Don S]

Re: Gearbox ratio and prop size?

Props are not the issue at this point. That prop should pop you up on plane, probably won't go very fast but it would sure plane the boat in a hurry.
Do as QC mentioned and verify the gear ratio. If it's wrong, changing props isn't going to do anything.

Also find out if he knows how to properly set the timing on a Volvo 3.0L with EST distributor, and if he has the shunt needed to set it.

 

[QC]

Re: Gearbox ratio and prop size?

How did it perform before the engine swap? Any data from that? You said "substantial repair"; what else was done to the hull etc.?

 

[fishrdan]

Re: Gearbox ratio and prop size?

Short answer - I don't know. It has only been tested by the mechanic this morning. Is operating the trim not something obvious that he should know about? Excuse my newbie ignorance!

Trimming up the drive is something "you" need to do while operating the boat. The mechanic can test the system to make sure it's working, but you need to trim it up-down while driving the boat. While coming out of the hole and breaking up on plane, trim the drive up a little bit.

I haven't "had to" trim the boat's I've owned, for max performance yes, but I could just leave them trimmed down and get on plane.

With a 1:97 drive ratio and 17P prop, it should easily pop up on plane. I used to have an old (waterlogged) 16.5' SeaSwirl with a 3.0L and it would easily plane with a 19P prop, with a 17P it would pin you to the seat and over-rev the engine.

If it won't go past 3K RPM, I'd check to see if the distributor is advancing properly and that the timing is set correctly. Is the engine surging or acting weird when it won't go past 3K RPM?

[ edit: didn't see DonS's post, great minds think alike, I guess ]

 

[scottydog78]

Re: Gearbox ratio and prop size?

I don't know how it performed - previous owner used it for cruising around rather than watersports or anything though we bought it cheaply knowing that it would need a new block as a minimum. Apart from the engine the only other changes have been cosmetic (cover repair, filling in a couple of minor dents etc). Nothing that should affect the performance of the boat. As I'm not sure how the boat was performing before I don't know whether the issue has come about due to the replacement or whether it was there before

How did it perform before the engine swap? Any data from that? You said "substantial repair"; what else was done to the hull etc.?

 

[scottydog78]

Re: Gearbox ratio and prop size?

Engine just holding back at 3k RPM rather than over revving or anything else - so less likely to be a prop issue. I think the suggestions of checking gear ratios, distributor and timing are worth checking out. Cheers!

 

[fishrdan]

Re: Gearbox ratio and prop size?

Apart from the engine the only other changes have been cosmetic (cover repair, filling in a couple of minor dents etc).

Explain further, were there any cracks or holes (even if small) in the hull?

 

[jeffnick]

Re: Gearbox ratio and prop size?

I ran a Seaswirl 1850 w/ 115 outboard and 17" prop. Didn't matter where the trim was set, she'd plane. I doubt there is any waterlogging. My money is on a motor/fuel problem. Filter clogged, fuel line pinched, water in the fuel, plug fouled, timing off, exhaust restriction, throttle linkage?