Trimming the leg and wot

[Warpa]

I have been messing around trying to 'get it right' for a few weeks now to no avail. My old outboard on my little dingy years ago was childs play, get the boat onto a plane then look at position of leg, move pin to adjust.

Maybe its the choppy Solent making things more difficult, when on a plane i trim up a few clicks and all that happens is engine revs go up, speed doesnt see to alter. I tend to go by conditions of the sea and my bow height and with the leg right down the bow is always at what i concider a good height so as not to bring on water.

With the leg fully lowered the waves break just behing the helm on a 17' boat generally with 3 people on board. Is it a case that a boat of that size in choppy water will not really benefit from adjusting the trim?

Another thing i noticed this weekend; while i normally travel at around at between 2 and 3k rpm i opened it up to 4k rpm heading back for the kids and it seemed to stay on top of the waves whereas at 2.5k rpm heading in the same direction we were riding them more than skimming over them.

This is with a mercruiser 3L and quoted top speed 40.8 and cruising speed 29.3, i have an ss prop so not sure if it differs from the original, engine wot is 4.4k to 4.8k rpm.

When at 4k rpm its the 1st time i have pushed the engine that hard, its fully serviced and looked after and i must say was a whole new experience in speed, oil pressure remained stable and its only at 3k rpm that the engine temp actually starts to go up.

Would i be right in thinking if i keep a check on the temp i can run wot for say 5 or 10 mins at a time? I cant find what the cruising speed rpm is on the net Or could i go wot from hamble to the iow without stressing the engine?

I dont want to cause any damage as im hoping to upgrade and that will go out of the window if i have engine rebuild costs

Sorry about the waffle, just want to get more of an understanding on how things work

 

[MapisM]

Would i be right in thinking if i keep a check on the temp i can run wot for say 5 or 10 mins at a time?

Short answer, no. By the time you will spot a raise in temperature or an alarm goes off, if you're running at WOT you might have already damaged the engine.
If your boat has a raw water pressure gauge (which only few boats have, actually), that's where you should constantly keep one eye, if you really must cruise at WOT for 10 mins - which is a long time!
But do you, really?
40+ (knots, I guess?) is a helluva speed with a 17 footer. I don't see any reason why you might wish to cruise at more that 30, if that.

Anyway, back to your original question, the outdrive trim is NOT meant to adjust the boat angle of attack. Trim tabs are what you should use for such purpose, and if you don't have any, you should consider fitting them.
I recently gave my view on that in this post and some others which followed on the same thread.

 

[tinkicker0]

I have been messing around trying to 'get it right' for a few weeks now to no avail. My old outboard on my little dingy years ago was childs play, get the boat onto a plane then look at position of leg, move pin to adjust.

Maybe its the choppy Solent making things more difficult, when on a plane i trim up a few clicks and all that happens is engine revs go up, speed doesnt see to alter. I tend to go by conditions of the sea and my bow height and with the leg right down the bow is always at what i concider a good height so as not to bring on water.

With the leg fully lowered the waves break just behing the helm on a 17' boat generally with 3 people on board. Is it a case that a boat of that size in choppy water will not really benefit from adjusting the trim?

Another thing i noticed this weekend; while i normally travel at around at between 2 and 3k rpm i opened it up to 4k rpm heading back for the kids and it seemed to stay on top of the waves whereas at 2.5k rpm heading in the same direction we were riding them more than skimming over them.

This is with a mercruiser 3L and quoted top speed 40.8 and cruising speed 29.3, i have an ss prop so not sure if it differs from the original, engine wot is 4.4k to 4.8k rpm.

When at 4k rpm its the 1st time i have pushed the engine that hard, its fully serviced and looked after and i must say was a whole new experience in speed, oil pressure remained stable and its only at 3k rpm that the engine temp actually starts to go up.

Would i be right in thinking if i keep a check on the temp i can run wot for say 5 or 10 mins at a time? I cant find what the cruising speed rpm is on the net Or could i go wot from hamble to the iow without stressing the engine?

I dont want to cause any damage as im hoping to upgrade and that will go out of the window if i have engine rebuild costs

Sorry about the waffle, just want to get more of an understanding on how things work

most efficient cruise rpm for merc 3.0l is 3000 rpm, at that setting you should be seeing around 5mpg and 30 knots assuming a 1:1.98 geared leg and a 21p prop.

 

[gjgm]

Maybe its the choppy Solent making things more difficult, when on a plane i trim up a few clicks and all that happens is engine revs go up, speed doesnt see to alter. I tend to go by conditions of the sea and my bow height and with the leg right down the bow is always at what i concider a good height so as not to bring on water.

Thats not really the idea, though. Forget about fitting trim tabs on 17ft- just use the leg you have.
I assume this isnt some racing boat, right, and outright performance isnt the goal?
You are trying to angle the bow to provide the best performance. That might be comfort, or speed, or a combo of both. Thats up to you.
Wind and tide will shape the wave, so it usually has a steeper face and a longer slope, depending which way you are looking at it!
Going into the sharp face of the wave, convention will be to push the bow down so the sharp point of your hull cuts the wave. With the seas from behind, you tend to lift the bow to support the bow as you go down the trough and approach the wave now in front. How effective this all is depends on your boat design.
As the sea is never the same, you just have to angle the leg to suit the day- and possibly each time you change course. On a short distance, you might enjoy the speed over comfort, but on a longer passage you might prefer a bit more comfort.
As regards skimming the waves, just draw some different "waves" on a piece of paper. It should be pretty clear why sometimes going faster will be better, and other waves it will be damn aweful !
The Solent is famous for its nasty short choppy waves-sometimes, its just the wrong sort of sea !

 

[Warpa]

Thats not really the idea, though. Forget about fitting trim tabs on 17ft- just use the leg you have.
I assume this isnt some racing boat, right, and outright performance isnt the goal?
You are trying to angle the bow to provide the best performance. That might be comfort, or speed, or a combo of both. Thats up to you.
Wind and tide will shape the wave, so it usually has a steeper face and a longer slope, depending which way you are looking at it!
Going into the sharp face of the wave, convention will be to push the bow down so the sharp point of your hull cuts the wave. With the seas from behind, you tend to lift the bow to support the bow as you go down the trough and approach the wave now in front. How effective this all is depends on your boat design.
As the sea is never the same, you just have to angle the leg to suit the day- and possibly each time you change course. On a short distance, you might enjoy the speed over comfort, but on a longer passage you might prefer a bit more comfort.
As regards skimming the waves, just draw some different "waves" on a piece of paper. It should be pretty clear why sometimes going faster will be better, and other waves it will be damn aweful !
The Solent is famous for its nasty short choppy waves-sometimes, its just the wrong sort of sea !

This is my reason for wanting to upgrade to a 19', not a great deal bigger but should let me spend more time around the IOW in 'normal' seas. Every video i sea of people boating around the island, it looks lovely and flat, even on the calmest days the solent is a bit of a mess with all the traffic, always have to keep an eye out for some oversized wash

 

[MapisM]

Forget about fitting trim tabs on 17ft- just use the leg you have.

Yeah, right.
So, if not on a 17 footer capable of 40+ kts, what's your idea of a boat which would benefit from trim tabs, exactly?

 

[Warpa]

Yeah, right.
So, if not on a 17 footer capable of 40+ kts, what's your idea of a boat which would benefit from trim tabs, exactly?

Trim tabs from what i have read will achieve great results, but that will be for the next owner to find out if thet want to fit them My main point ( i think) was that trimming the leg doesnt appear to alter the bow height in any way.

I have only trimmed it up a few nothces, maybe i should take it further untill i get prop spin, then lower to suit

 

[MapisM]

Well, if as you previously said you already trimmed up till revs increase, with no effect on speed, I'm afraid there's not a lot you can do to further raise the bow. That's also the only situation where trim tabs wouldn't help, btw.
This is pretty unusual for a small speedboay, though. Do you have by chance some unusual weight stored up front (heavy anchor, or anything else)?

 

[Solitaire]

I have been messing around trying to 'get it right' for a few weeks now to no avail. My old outboard on my little dingy years ago was childs play, get the boat onto a plane then look at position of leg, move pin to adjust.

Maybe its the choppy Solent making things more difficult, when on a plane i trim up a few clicks and all that happens is engine revs go up, speed doesnt see to alter. I tend to go by conditions of the sea and my bow height and with the leg right down the bow is always at what i concider a good height so as not to bring on water.

With the leg fully lowered the waves break just behing the helm on a 17' boat generally with 3 people on board. Is it a case that a boat of that size in choppy water will not really benefit from adjusting the trim?

Another thing i noticed this weekend; while i normally travel at around at between 2 and 3k rpm i opened it up to 4k rpm heading back for the kids and it seemed to stay on top of the waves whereas at 2.5k rpm heading in the same direction we were riding them more than skimming over them.

This is with a mercruiser 3L and quoted top speed 40.8 and cruising speed 29.3, i have an ss prop so not sure if it differs from the original, engine wot is 4.4k to 4.8k rpm.

When at 4k rpm its the 1st time i have pushed the engine that hard, its fully serviced and looked after and i must say was a whole new experience in speed, oil pressure remained stable and its only at 3k rpm that the engine temp actually starts to go up.

Would i be right in thinking if i keep a check on the temp i can run wot for say 5 or 10 mins at a time? I cant find what the cruising speed rpm is on the net Or could i go wot from hamble to the iow without stressing the engine?

I dont want to cause any damage as im hoping to upgrade and that will go out of the window if i have engine rebuild costs

Sorry about the waffle, just want to get more of an understanding on how things work

I have sent you a PM.

 

[gjgm]

Yeah, right.
So, if not on a 17 footer capable of 40+ kts, what's your idea of a boat which would benefit from trim tabs, exactly?

Exactly? I dont think there is going to be an exactly. OP says he can lift the bow so high that there is no risk of water coming in. The boat isnt going to be effected by lean with a side wind, so I would bother with trim tabs.
I dont see why it being 17ft and 40 knots (in lake conditions, I am guessing)means it needs tabs.
But, as I did say, I assume we are not talking about some race boat here, but a smaller runabout.
I cant think I have ever seen a 17-18ft sportsboat with tabs, so I guess not everyone agrees with your opinion on this.

 

[MapisM]

I dont see why it being 17ft and 40 knots (in lake conditions, I am guessing)means it needs tabs.

Well, you obviously didn't bother reading what I wrote in the other thread which I previously linked.
Fairenuff anyway, you can think what you please of course.
But with regard to the fact that tabs are rarely fitted to small boats, I'm afraid the reason is not because the builders and customers disagree with my opinion. It's waaaay simpler than that, just think about it.

 

[duncan]

Well, you obviously didn't bother reading what I wrote in the other thread which I previously linked.
Fairenuff anyway, you can think what you please of course.
But with regard to the fact that tabs are rarely fitted to small boats, I'm afraid the reason is not because the builders and customers disagree with my opinion. It's waaaay simpler than that, just think about it.

1. price impact
2. too much opportunity for people to get it dangerously wrong - also why the trim in angles are carefully limited by the manufacturers even though a few more degrees would be better for many specs.
3. another thing to go wrong - warranty costs issue
4. trim tabs and trailers don't mix, especially when there are different US and EU trailer specs for the same model
5. trim tabs and transom mounted transducers (common to this size craft) need care and can be a real issue with other hull elements ie the tabs will want the clear water that the transducer would want.
6. such craft will run with almost no hull in the water at WOT, and in many markets they are only run at high speed!
7. despite 6 above the existence of any tab structure (unless inset in the hull at huge cost (new molds etc etc) will reduce the max speed and increase fuel consumption at high speeds (they rarely produce the theoretic improvements associated with running the drive in perfect alignment anywhere above hull speed + 100% either) on such craft.

but the reality is it's not cost effective ie the gain doesn't outweigh the cost - which actually makes gjgm's underlying case! You can achieve most of what you would use tabs for on a larger craft through moving a coolbox or crew and drive trim on a small one. In fact adjusting weight distribution is a better way because it doesn't introduce drag.

 

[gjgm]

and
you dont need them

 

[Warpa]

1. price impact
2. too much opportunity for people to get it dangerously wrong - also why the trim in angles are carefully limited by the manufacturers even though a few more degrees would be better for many specs.
3. another thing to go wrong - warranty costs issue
4. trim tabs and trailers don't mix, especially when there are different US and EU trailer specs for the same model
5. trim tabs and transom mounted transducers (common to this size craft) need care and can be a real issue with other hull elements ie the tabs will want the clear water that the transducer would want.
6. such craft will run with almost no hull in the water at WOT, and in many markets they are only run at high speed!
7. despite 6 above the existence of any tab structure (unless inset in the hull at huge cost (new molds etc etc) will reduce the max speed and increase fuel consumption at high speeds (they rarely produce the theoretic improvements associated with running the drive in perfect alignment anywhere above hull speed + 100% either) on such craft.

but the reality is it's not cost effective ie the gain doesn't outweigh the cost - which actually makes gjgm's underlying case! You can achieve most of what you would use tabs for on a larger craft through moving a coolbox or crew and drive trim on a small one. In fact adjusting weight distribution is a better way because it doesn't introduce drag.

When i have been out with 5 adults it was probably the most comfortable ride i have had, 4 in the main part of the boat and a 14 yo nipper in the bow, that was more down to weighting the whole boat down as opposed to moving the punters around inside

I will go down the side of the docks this weekend to the ski area where the water is always flat and have a play with the tabs there at 3k and wot to get a proper feeling of how much the bow is being raised and lowered....will report back

 

[MapisM]

1. price impact

Give the man a cigar!
Though I'm afraid you're totally missing the whole point of tabs with your point 7.
Otoh, an explanation of why tabs can also improve max speed (hence obviously reduce fuel consumption) would be way o/t.
And tbh, neither I'm interested to convince you or anyone else.

Btw, I actually agree that trim tabs are not needed on the OP boat. Where did I say they are?
Arguably, we don't really need pleasure boats at all, or do we?

 

[duncan]

Give the man a cigar!
Though I'm afraid you're totally missing the whole point of tabs with your point 7.
Otoh, an explanation of why tabs can also improve max speed (hence obviously reduce fuel consumption) would be way o/t.
And tbh, neither I'm interested to convince you or anyone else.

Btw, I actually agree that trim tabs are not needed on the OP boat. Where did I say they are?
Arguably, we don't really need pleasure boats at all, or do we?

we can all play with words -

I totally agree that tabs can improve max speed, and that they can definitely improve fuel consumption (especially within the range 'hull speed -> 2x hull speed).

However I am also of the view that they can reduce max speed and increase fuel consumption figures (which esp in the US are given at specific speeds). You don't seem to accept that view.

I am sure we could also have the same debate over why outboards are fitted with 3 blade props.....but let's not

 

[MapisM]

However I am also of the view that they can reduce max speed and increase fuel consumption figures (...). You don't seem to accept that view.

Hang on, in your previous post you said that the existence of any tab structure (...) will reduce the max speed and increase fuel consumption at high speeds.
This was the sweeping generalization which I didn't accept, because in some boats - mine included - exactly the opposite is true.

Otoh, if you're now saying that they CAN (rather than WILL) make things worse, I'm with you.
Trim tabs, if poorly installed and/or not properly matched with the hull, of course can be useless.

With regard to why O/Bs are fitted with 3 blade props, does this answer the question?