Weight distribution for WOT testing?

[tmh]

I have a new-to-me 1985 Mark Twain 19' Open bow that weight about 2500. V8, 200hp (at least rated that - who knows what it really puts out 21 yrs later). I've never inspected prop as I bought the boat in the water and fully plan to replace it once I know what to get. I'm told it's a 19" 3-blad aluminum - I'd suspect not in quite-new condition.

I'm going out to do some of this WOT testing I learned about here for future prop selection. I'm wondering where my passengers should be seated for this?

I have 3 kids avg. wt. about 75 lbs (225 lbs total) who usually ride in the bow. My wife (125 lbs) somewhere in back and i drive. We plain out fine about 20 mph and it cruises fine 20-35. but top end is high 30s which seems slow for a 19 footer (a heavy one, i know) with 200hp; about 3800rpm WOT, if it's accurate. It is a 2-bbl which i understand gives less power than a 4-bbl.

So, where to sit the kids for testing and what else to test to help figure out which prop to try?

 

[xtraham]

Re: Weight distribution for WOT testing?

if the boat has been in the water for any length of time you will need to pull it and clean the bottom before the results of any good test, rpm's decrease drasticly if the bottom is fouled, start with a clean bottom and a light load, then the experts here will guide you to the proper set up, good luck

 

[tmh]

Re: Weight distribution for WOT testing?

ahhh.......that makes lots of sense. I'm sure the hull is likely covered in crap. I won't be taking it out again for a few weeks, so I'll just enjoy it for now and worry about WOT and a new prop later on. Thanks for the input.

 

[Dhadley]

Re: Weight distribution for WOT testing?

On a recreational boat we always test with an average load. Nothing special. If you set up for something very specific and then vary that in your everyday cruise, you may not be getting all the efficiency that's available.

Define your intended use and test along those guidelines.

 

[Plainsman]

Re: Weight distribution for WOT testing?

Is 3800 RPMs all your getting? Or is that where you cruise it?

 

[tmh]

Re: Weight distribution for WOT testing?

The 3800 was what I was seeing WOT and 35-37 mph. That was with 3 - 75lb kids in the bow, me and my wife (310 lbs combined) behind the winshield, pretty full tank.

Today, by myself with 2/3 tank I got to about 4100 and 40 mph. It rides totally different without the kids up front. Takes a while to plain (until I get me Smart Tabs) and rides up a bit in front - likely why I got some more speed out of it.

This is a boat I bought in the water so the hull likely has lots of gunk growing on it. The prop is used, but I don't know the condition. Too many variables right now so I'll do some real testing when I pull the boat out in a few weeks, clean it, check the prop out, etc.

 

[tmh]

Re: Weight distribution for WOT testing?

OK. I'm doing some testing in the next week or so. Trying some lower pitch props to see where the WOT goes. (I have another thread on the prop selection because I forgot about this thread).

What about weight distribution? My 3 kids, as mentioned earlier, and sometimes a friend of theirs (or 2) just love the bow seats and never seem to want to move back except when we are tubing or something. Do I leave them up there for prop testing even though my boat performs better with a couple of them in back?

The boat is all about the kids having fun (and me wanting to maximize how it runs) so I won't be having them ride in back "for efficiency sake".

Thanks.

 

[Crownie2]

Re: Weight distribution for WOT testing?

This is what power trim is all about. I assume you have it. When the kids are up in the bow you trim up and that raises the bow and compensates.

Bob

 

[tmh]

Re: Weight distribution for WOT testing?

Yes, i have power trim. However, I am talking about how to best test for a prop. Trim or not, the boat rides different with weigh in the bow vs. the back. My bow stays down when i have 3-5 kids up there even with the prop trimmed out as far as the water lets me. Maybe a "proper" prop will help with this as well. Could be more RPMs does the trick for getting the bow up a bit more when on plane?

 

[JasonJ]

Re: Weight distribution for WOT testing?

That is exactly why you use the trim, to try to compensate for the weight up front by trimming out, thus lifting the bow. Also, although I am no expert, there are props that help to give additional lift to the bow.

Before you start throwing props at the boat, make sure the engine is in proper tune and producing all of its power. No prop is going to make up for a poorly tuned engine. You also want to do the basic checks like compression testing each cylinder.

If the motor is in top notch tune, and the bottom is clean, then it is time to look at load distribution. If people insist on sitting up front, then you need to move heavy stuff back. Coolers, anchors, batteries, all of that is moveable weight.

If you have optimized weight distribution, engine tune, bottom condition, and have trying everything with your trim, then look at props. Just bear in mind that sometimes you just can't make up for a bunch of carcasses in the front of a boat, and you might just have to settle for what you have. Good luck....

 

[Crownie2]

Re: Weight distribution for WOT testing?

I am a little curious as to why you think you need the Smart Tabs.

Don't forget they may tend to push the bow down a little, depending on how they are adjusted - just what you don't want with the kids up front... And if you adjust them so they don't have that effect, they may not do much of anything...

I'd work with that power trim a little more first.

And yes, many good props do have bow lifting ability because that usually adds some speed.

Bob

 

[tmh]

Re: Weight distribution for WOT testing?

Bob, good question as to why the smart tabs. I read many posts in several forms regarding Smart Tabs and came to the conclusion they do several things:

1. Improved hole shot with a less-raised bow. My boat is heavy for it's size and has a heavy engine for a 19' so unless I'm very front heavy with kids I need some help out of the hole. For skiing, tubing and parasailing I won't have the kids up there.

2. Reduced cruising speed on plane. My wife likes to putter around a bit so this is good.

3. Improved handling in "no wake" areas. By boat has quite pronounced "bow steer" which is a pain when going slow, especially docking. When in shallow water (where I currently dock) I have to trim out a bit and then it's worse.

Your point of pushing the bow down with kids up there is a concern I have with the Tabs. However, they are pressure weighted so as to produce less stern lift the faster the boat is going. Also, I bought the "60 lb" ones even though my boat would spec at 80 lb tabs because I figure these will flatten out quicker so as to have less influence when on plane.

I'll let folks know how the Smart Tabs work out. I must say though, I've rarely seen a product get so close to 100% approval from actual users as i see with these things. Mostly, it was exactly my problems folks say the tabs fix. To solve my "too many kids in the bow" problem, maybe I'll need to get some video games set up in the stern!

 

[Crownie2]

Re: Weight distribution for WOT testing?

OK, great - sounds like you've done your homework.

Keep us posted.

Bob

 

[tmh]

Re: Weight distribution for WOT testing?

Hey, one more question if I may.... The prop that is on the boat I just bought is a 19" Alum 3-blade. it is a bit nicked up but I don't know how much wear it needs to show before noticable performance issue are seen. It doesn't look bent or dented to my untrained eye, just a few nicks at the tips and marks on the surface of the blades.

If that type of wear DOES have an effect, what's it likely to do to WOT RPM and MOH? I assume reduce each slightly, correct?

 

[Crownie2]

Re: Weight distribution for WOT testing?

Sounds correct. You can have it repaired to like new condition...

Bob