More top end?

[Fenlander]

Re a position for the small tach... I just cable tie one to the tiller where I can see it for my initial engine and prop testing when I change outboards. Then once sorted I relocate it to under the hood to use as an hour meter and rev counter when servicing.

[Fenlander]

Under hood after testing on my current OB.... and second image on previous OB.

This is the tach type I prefer... not quite the £10 or less of some but no more than £20 and I prefer they have an earth wire and readings seem more stable for it.

[Windsurfer]

I was hoping it was ok with the Av plate set below the hull abit , how far above my hull should I set the Av plate , just reluctant to drill holes and it doesn’t make any difference.
I’ve read around 20mm above is a good height?when I use a straight edge to show the height difference it obviously changes depending on the setting of the outboard , none of the settings will hold the Av plate exactly parallel to the hull , it either slightly angled up or down.
Should I just use a block to support the outboard between pin holes to get the Av plate parallel then measure from there and adjust accordingly?
Thanks

[Windsurfer]

Originally Posted by Limecc

Here's a transom drilling template. My holes are drilled black and yellow which allows me to run my AV plate +40mm above the hull in the highest setting, two holes down is level and it's -20mm below on the lowest.

I think you will be safe starting 20mm higher than level and if you are lucky it still won't ventilate in the highest position.

I’ve just seen this and answers my height question above , great info and pictures really helpful

[beamishken]

Originally Posted by Windsurfer

I was hoping it was ok with the Av plate set below the hull abit , how far above my hull should I set the Av plate , just reluctant to drill holes and it doesn’t make any difference.

I’ve read around 20mm above is a good height?when I use a straight edge to show the height difference it obviously changes depending on the setting of the outboard , none of the settings will hold the Av plate exactly parallel to the hull , it either slightly angled up or down.

Should I just use a block to support the outboard between pin holes to get the Av plate parallel then measure from there and adjust accordingly?

Thanks

I normally start 20 -25mm above, thats where I'd start, if your drilling holes I'd drill so the engine can be mounted level if required so you have the maximum adjustment upward, you'll never need to go deeper than level. It also means once lifted you probably won't have the bolts in the bottom hole which always looks a bit precarious. Don't rely on prop calculations, far too many variables to be 100% accurate, a tach & wot trails with different props are the answer.

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[Limecc]

Originally Posted by Windsurfer

Should I just use a block to support the outboard between pin holes to get the Av plate parallel then measure from there and adjust accordingly?

Yes measure it parallel and double check the template holes will be in the right place. In the absence of hydraulic tilt when on the water you'll have to experiment which pin position works best for each height setting.

The top holes are fixed, you can't drill too close to the top edge of the transom, I'd stick to the 1.7/8" of the standard template but you have a bit of variance so you can set it like Ken says. Each transom will have differences.

Ordinary silicon sealant will do until you've decided where the engine's going to go then switch to Polyeurethane.

P.S. Raising two holes transformed my current boat you won't believe the difference.

[chipko]

Can’t help with prop pitch unless you know revs at wot so, as Fenlander correctly points out, get a cheapo unduction tach and take it from there.
All this later advice re height all sounds a bit ott to be honest! Is your 25 currently bolted to transom or just clamped? If clamped just add a 10-20mm packer and go test.
It may be a little low and worth raising level with keel, which might reduce splashing, but performance gains will be minimal.
Bear in mind this is a lightweight (circa 75kg) 3.8 alumin shallow V hull, so running av plate 20, 30, 40, 50mm etc. above keel probably won’t work and ventilate like crazy. OK on a 750kg+ rib where 50mm+ is common, but not at this weight.
Our 3.4/20hp lightweight combo runs best with AV plate level with keel, no splashing and only minimal ventilation in tight turns.





Looking at your pics, my only comment would be the motor is trimmed in too much. Try and get the AV plate level with water at planing speeds and at least in line with keel. This may even have more effect on top speed than simply raising motor by 20mm.

[Limecc]

Thing is, he already said he can't raise it by more than 10mm using the clamps. I'm firmly in the bolt-em-up camp, better to be safe than sorry.

Also, his V looks to have a decent depth to it, not a shallow V.

[chipko]

Yet again have to question your wisdom.
Is your advice based on experience with sibs/small lightweight shallow V ribs of this size.
I think not!!

[Limecc]

Originally Posted by chipko

Yet again have to question your wisdom.
Is your advice based on experience with sibs/small lightweight shallow V ribs of this size.
I think not!!

Correct my advice is of my own rib and hard sided boat experience. It's a deeper V than mine. I humbly acknowledge your sibbing wisdom. The pic you posted is a shallow V. Does it even have one?

[chipko]

Yes correct, the pic I posted is a shallow V
Does it have any V? Yes it does, terminating with a planing pad at the stern.



If you think a Quicksilver 380 alumin is a deep V hull then not sure what planet you’re from there Matey.

[Limecc]

Originally Posted by chipko

If you think a Quicksilver 380 alumin is a deep V hull then not sure what planet you’re from there Matey.

Please read where I said it has a deeper V than mine. NOT that it is a deep V. I'd class mine as a medium. Cheers.

[Windsurfer]

Originally Posted by Fenlander

Under hood after testing on my current OB.... and second image on previous OB.

This is the tach type I prefer... not quite the £10 or less of some but no more than £20 and I prefer they have an earth wire and readings seem more stable for it.

Thanks Fen , ive just bought the same looking one.Does it just wrap around the spark plug lead and that’s it?

[chipko]

All somewhat theoretical what we perceive as shallow, medium or deep V.

Bottom line is I would never apply the same set up criteria to a larger rib and small lightweight sib/rib. None of our small lightweight sibs/ribs have ever run well with the AV plate higher than the keel. Our 650kg+ rib runs well and benefits with AV plate 50mm above keel. Also there is a massive difference having the luxury of ptt rather than just manual trim when deciding optimum engine height.

For the OP still say get a tach to determine wot revs before swapping props and moving up a few mm or so will have minimal effect on top speed. Anywhere between level and 25mm below keel performs ok on most hulls, although splashing is significantly reduced the higher you can go. Also don’t forget trim.

[Windsurfer]

Originally Posted by chipko

Can’t help with prop pitch unless you know revs at wot so, as Fenlander correctly points out, get a cheapo unduction tach and take it from there.
All this later advice re height all sounds a bit ott to be honest! Is your 25 currently bolted to transom or just clamped? If clamped just add a 10-20mm packer and go test.
It may be a little low and worth raising level with keel, which might reduce splashing, but performance gains will be minimal.
Bear in mind this is a lightweight (circa 75kg) 3.8 alumin shallow V hull, so running av plate 20, 30, 40, 50mm etc. above keel probably won’t work and ventilate like crazy. OK on a 750kg+ rib where 50mm+ is common, but not at this weight.
Our 3.4/20hp lightweight combo runs best with AV plate level with keel, no splashing and only minimal ventilation in tight turns.


Attachment 140047


Looking at your pics, my only comment would be the motor is trimmed in too much. Try and get the AV plate level with water at planing speeds and at least in line with keel. This may even have more effect on top speed than simply raising motor by 20mm.

The tach as been ordered and will be the first test , I’ve only really got around 8mm left on the clamp plate but that would be my second test once I can get some readings from the tach.

That picture was when I first picked up the boat and set the outboard on the first pin hole , I found I worked best trimmed out around on the 3rd or 4th hole.
I can’t get the Av plate exactly parallel with the keel it’s either slightly angled up or down between hole 3 and 4.
Just need some calm weather for testing now

[jeffstevens763@g]

Originally Posted by chipko

Can’t help with prop pitch unless you know revs at wot so, as Fenlander correctly points out, get a cheapo unduction tach and take it from there.
All this later advice re height all sounds a bit ott to be honest! Is your 25 currently bolted to transom or just clamped? If clamped just add a 10-20mm packer and go test.
It may be a little low and worth raising level with keel, which might reduce splashing, but performance gains will be minimal.
Bear in mind this is a lightweight (circa 75kg) 3.8 alumin shallow V hull, so running av plate 20, 30, 40, 50mm etc. above keel probably won’t work and ventilate like crazy. OK on a 750kg+ rib where 50mm+ is common, but not at this weight.
Our 3.4/20hp lightweight combo runs best with AV plate level with keel, no splashing and only minimal ventilation in tight turns.


Attachment 140047


Looking at your pics, my only comment would be the motor is trimmed in too much. Try and get the AV plate level with water at planing speeds and at least in line with keel. This may even have more effect on top speed than simply raising motor by 20mm.

Agree with you on this trim can easily kill the performance and a tacko is a must before any of the other advice.

[Limecc]

There we go then. Unlike Chipko I would make no assumptions where the AV plate ought to be for this hull. I believe he's right if it were a soft bottomed sib. For me the principle is always the same. 1. Optimise engine height. 2. Optimise prop.

Ken is spot-on about extra drag from an engine that's too low. Sure, can optimise the prop taking that extra drag into account or make sure it's as little as it needs to be. I found out this was worth a pitch or two of prop. For free right?

On my Mercury (Quicksilver?) 420 rib I was totally disappointed upgrading 30 to 40 hp. Why? A long experienced marina mechanic set the AV plate 25mm below the hull. Wasted most of the extra power. Most noticeable on the top end.

Both Jeff and Chipko are right that trim can kill performance, it will be in addition to the extra drag.

Anyway it's a moot point. To raise the AV plate up level where Chipko has his set, need to drill the transom.

[beamishken]

Originally Posted by Limecc

There we go then. Unlike Chipko I would make no assumptions where the AV plate ought to be for this hull. I believe he's right if it were a soft bottomed sib. For me the principle is always the same. 1. Optimise engine height. 2. Optimise prop.

Ken is spot-on about extra drag from an engine that's too low. Sure, can optimise the prop taking that extra drag into account or make sure it's as little as it needs to be. I found out this was worth a pitch or two of prop. For free right?

On my Mercury (Quicksilver?) 420 rib I was totally disappointed upgrading 30 to 40 hp. Why? A long experienced marina mechanic set the AV plate 25mm below the hull. Wasted most of the extra power. Most noticeable on the top end.

Both Jeff and Chipko are right that trim can kill performance, it will be in addition to the extra drag.

Anyway it's a moot point. To raise the AV plate up level where Chipko has his set, need to drill the transom.

Yep you hit the nail on the head the ops boat has far more vee than the other boat pictured which looks almost flat bottomed. Lifting the engine is free as you say & will likely gain revs as it goes up until it gets to the point of excess ventilation at which point I'd probably drop back one hole & your usually as good as it gets. Once you've maxed out revs with trim and height adjustments then look at switching props at £100 a pop.
Just as an experiment try sticking your fingers an inch below the water over the side at wot & see how much pressure it takes to keep your hand there. getting the av plate above the hull & out of the water flow behind the boat removes a huge amount of resistance which converts to improved performance

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[Scooby]

I'm about to mount my engine tomorrow started yesterday on trolley in a dustbin how much water do you get from pee hole on your 25 bmh please as it's different to my Yamaha 15 just wanted to know if normal or not
Thanks

[chipko]

Ok, also a big advocate of raising motors as high as practical until ventilation becomes an issue. I get that, and agree the OP’s motor is a little low, but only a little. Sounds like it can be easily raised 10mm and still clamped on so would definitely try that.
Gut feeling on that hull it could probably benefit by going up by around 25mm but that would mean abandoning clamps and drilling transom to bolt on. Saddles on portable motors aren’t usually the ISO pattern standard found on larger motors with five or so height choices available after drilling. For that reason I would not be advocating this solution unless absolutely 100% sure of height…it’s a one shot solution. Even if you get it right it’s only applicable to that motor.
It’s not as if the motor is 50mm or so too low where performance would be seriously affected. Going up from where it’s at now will offer some performance gains, but only marginal ones.
Raise the easy 10mm, fit tacho, go testing, adjust trim and ascertain suitable prop pitch first would be the way to go.
Good luck.