Outboard mounting height?

[wipe_out]

Hi,

Last time we were out I looked over the transom while we were cruising at about 25kn (max speed about 35kn) and the bottom of the outboard was very much under water with no sign of the cavitation plate.. In fact the next plate up (think its called the splash guard or something) was pretty much at the water line..

From what I have been able to find online it seems the perfect outboard height is where the cavitation plate is pretty much skimming the surface but what I haven't been able to find is whether this should only happen at full speed or if it should happen when planing at reasonable speed..

I probably need to get out again on a calm day and have someone take us up to full speed while I have a look over the back or rig up a camera to have a look..

Any experts around that can offer some advice on what the correct mounting height for an outboard is?

Thanks

 

[AndieMac]

No expert, but from what I've been told, essentially the cav plate should be level with the lowest point of the keel at the transom.

 

[Spi D]

As a rule of thumb the plate should be on level with the keel line. The height, aka X-dimension, however can be different for specific reasons like extreme speed where the legs resistance (drag) in the water 'hold you back'.

Certain transom designs benefit from a higher fitting too, eg.

To facilitate the optimal trim racers put a parallel lift on the transom, then the engine on the lift. Commonly powered and operable while underway.



When raised to an extent where the propeller breaks the surface, purpose made props are needed.

At 35 knots however, I presume none of these gizmos are relevant.

Be warned, that the return of investment on these toys is very low but your existing setup may allow for testing a higher position by using lower bolt holes on the bracket. Definately cheaper but would take a liftout plus means to lift & hold the engine.




If you want to know more feel free to download
http://spidybot.com/prop/Quicksilver_Propellers.pdf

 

[LadyJ]

This point has been raised following a post I made re my skeg being partly in the water when fully raised. Someone said, like you, that the plate should be almost out the water at planing speed. I checked and mine is slightly below and the plate sits level with the keel line like Spi D has said. I had raised this with the mechanic at the brokerage in relation to raising mine so the skeg was out the water, he assured me the correct depth was level with the keel and my skeg in the water issue wasn't a real problem but was as a result of the hydronic steering mechanism.
I'm a 755 with a Suzuki, so I think similar to your set up, and after the winter before I put everything back in the boat (donuts, wet suits etc) I got 34 knots out of her and the bottom hasn't been cleaned in 12 months and she sat in the water all winter. Reading the spec for the 755 I don't see me getting faster if I raised the engine? Interested to see other views on your question.

 

[Spi D]

If you find it worth the money a jack plate can of course be fitted alone to get the skeg out of the water when tilted up.

Many boats have your problem - the only actual risk being that growth may occur on whatever little is submerged. You will see boaters simply leaving the leg fully down while moored.

 

[LadyJ]

If you find it worth the money a jack plate can of course be fitted alone to get the skeg out of the water when tilted up.

Many boats have your problem - the only actual risk being that growth may occur on whatever little is submerged. You will see boaters simply leaving the leg fully down while moored.

Thanks Spi D, I think I'm just going to put some antifoul on the part that's submerged (4 inches) when it's out for a service and live with it. It irritated me at the start but when I'm at the boat the outboard is fully down so become irrelevant. Just new boat fussiness I suspect.

 

[Spi D]

For your entertainment this is a centre console Enes Explorer 570 in optimal trim with jackplate and stainless 3-blade prop. Balancing perfectly, kept on track by an experienced driver.

Doing 60 knots:


Notice how it runs only on the very back of the hull

Sorry for the vid quality.

 

[LadyJ]

:encouragement:

 

[wipe_out]

Thanks for the comments and videos..

I am not looking for a higher top speed but more at the possibility of reducing the drag which would improve fuel efficiency slightly.. Like LadyJ its probably new boat fussiness..

My thinking was more along the lines of shifting the engine up one bolt hole on the transom.. We are on a trailer so the lift out isn't an issue and we are due a service so was thinking about getting it done then..

Found this article and while it mentions gaining speed there are some interesting points..
http://www.boats.com/how-to/the-outboard-expert-boost-speed-with-outboard-engine-height-adjustments/

Seems its a bit of a black art like prop selection..

Also found this..

 

[Spi D]

If not at big job I'd suggest you go 'trial and error'. No harm if not at success and easily reversed.

 

[gordmac]

Look on the world sea fishing .com boat owning site, a common topic. Helpful too so worth asking.
Anyone know how to stop a tablet replacing what you want with predictive crap?

 

[VicS]

Hi,

Last time we were out I looked over the transom while we were cruising at about 25kn (max speed about 35kn) and the bottom of the outboard was very much under water with no sign of the cavitation plate.. In fact the next plate up (think its called the splash guard or something) was pretty much at the water line..

From what I have been able to find online it seems the perfect outboard height is where the cavitation plate is pretty much skimming the surface but what I haven't been able to find is whether this should only happen at full speed or if it should happen when planing at reasonable speed..

I probably need to get out again on a calm day and have someone take us up to full speed while I have a look over the back or rig up a camera to have a look..

Any experts around that can offer some advice on what the correct mounting height for an outboard is?

Thanks

The antiventillation plate should be level with the bottom of the boat transom

Your owners manual should have details . You will find the above diagram in many manuals

 

[wipe_out]

The antiventillation plate should be level with the bottom of the boat transom

Your owners manual should have details . You will find the above diagram in many manuals

Had a dig through the manual and cant find anything.. Looks like I need to do some testing and then see if I think its worth raising it one hole on the mounting..

 

[AndieMac]

Had a dig through the manual and cant find anything.. Looks like I need to do some testing and then see if I think its worth raising it one hole on the mounting..

If raised too high, probably good for WOT running in flat conditions, it can create a cavitation effect, basically the prop "let's go" during sharp turns and following seas, which will really annoy you.

 

[LadyJ]

If raised too high, probably good for WOT running in flat conditions, it can create a cavitation effect, basically the prop "let's go" during sharp turns and following seas, which will really annoy you.

Good point, I am aligned to the keel and even then do experience some minor cavitation in a following sea so would suggest a further raise will cause some frustration in anything but flat calm.

 

[sprocker]

I'm going through exactly this at the moment with my MF645 with Suzuki DF115. I have done a lot of research, and listened to lots of advice, some good, some not so good.

I am now in touch with the techies at Suzuki who maintain that the only correct way to set your engine height is at speed with the engine correctly trimmed. They suggest that 'at speed' should be WOT, but a good idea can be gained from high cruising speeds.

The anti ventilation plate should be on the surface and being splashed, and not underwater as this will cause drag, reducing effectivenes, cause lower fuel consumption and reduce engine longevity. (Suzuki engines are meant to rev, I would assume it's the same with other makes?).

If you place some red tape,/stickers on top of the anti vent plate, it will show op better on your video.

I sent some pics to the techies and they have suggested that my av plate is too 'buried', so I am going to lift one hole and see if it is any better.

 

[AllanG]

I'm going through exactly this at the moment with my MF645 with Suzuki DF115. I have done a lot of research, and listened to lots of advice, some good, some not so good.

I am now in touch with the techies at Suzuki who maintain that the only correct way to set your engine height is at speed with the engine correctly trimmed. They suggest that 'at speed' should be WOT, but a good idea can be gained from high cruising speeds.

The anti ventilation plate should be on the surface and being splashed, and not underwater as this will cause drag, reducing effectivenes, cause lower fuel consumption and reduce engine longevity. (Suzuki engines are meant to rev, I would assume it's the same with other makes?).

If you place some red tape,/stickers on top of the anti vent plate, it will show op better on your video.

I sent some pics to the techies and they have suggested that my av plate is too 'buried', so I am going to lift one hole and see if it is any better.

Surely, trying to set-up your engine this way is totally dependent upon the boat being at the optimum trim at WOT which, IMHO, is pretty impossible, as you can never guarantee to have the loaded weight of persons, fuel, stores, etc, etc, in the same positions each time you use your boat. Maybe this method of setting-up an outboard is fine for an out and out racing boat, but it seems a bit OTT for a 'cruising' boat.

Have a Honda outboard, and they set-up the outboard with the anti-cavitation plate level with the bottom of the keel, which appears to be the most common method.

 

[wipe_out]

I'm going through exactly this at the moment with my MF645 with Suzuki DF115. I have done a lot of research, and listened to lots of advice, some good, some not so good.

I am now in touch with the techies at Suzuki who maintain that the only correct way to set your engine height is at speed with the engine correctly trimmed. They suggest that 'at speed' should be WOT, but a good idea can be gained from high cruising speeds.

The anti ventilation plate should be on the surface and being splashed, and not underwater as this will cause drag, reducing effectivenes, cause lower fuel consumption and reduce engine longevity. (Suzuki engines are meant to rev, I would assume it's the same with other makes?).

If you place some red tape,/stickers on top of the anti vent plate, it will show op better on your video.

I sent some pics to the techies and they have suggested that my av plate is too 'buried', so I am going to lift one hole and see if it is any better.

Ahhh.. Now that is some interesting info.. What I couldn't find is what speed I should be running at to test the AV plate.. To me logically it would be WOT but some sites suggested cruising speed..

Also some info I read last night suggested the cavitation plate should be 1" above the keel for every 12" from the transom, so at 6" from the transom your cavitation plate would need to be about 1/2" above the keel and so on.. Was planning of going to do some measuring today but it's raining so maybe another day..

Will be interested to hear your results.. When are you planning in lifting the outboard?

 

[MapisM]

Seems its a bit of a black art like prop selection.

Yep, and the very simple reason why it seems so is that it really is.

I mean, the short (and in principle correct) answer to your question is the one you got in the first reply from AndieMac.
Otoh, you can find debates about X-dim all over the web (particularly in some US forums mostly aimed at fast boats) because depending on what your priorities are, you might achieve some improvements with a more "aggressive", as it's usually called, X-dim (more aggressive meaning higher).
As a rule of thumb, the higher the X-dim, the higher the top speed and the lower the maneuverability/holeshot acceleration/steerability.
Of course, there's a limit - specific to any hull/engine combo - to how much you can increase the X-dim, because above that limit the prop can either not have enough grip to bring the boat up on the plane, or cavitate even at speed, hence actually reducing rather than increasing top speed.
But it's only worth testing and fine tuning the X-dim with very fast hulls, capable of 50+ kts.
In fact, for these boats the difference between a buried and a raised leg can be up to a 10% top speed increase, if not more.

Bottom line, I wouldn't bother testing a lot with a 35 knots boat, because it's very unlikely that you can make her more than a knot or so faster, with a better fuel efficiency (if any) that you couldn't even appreciate, particularly at a cruising speed of 25 knots. And this ain't worth (imho) compromising the other previously mentioned factors.
Otoh, coming back to your original question, the reason why I would actually try to raise the o/b a bit in your case is that 25 knots is definitely a steady planing speed, at which the cavitation plate should NOT be buried in the water to the point of being invisible, assuming that the o/b is trimmed properly.
If with the boat on the trailer you align a ruler (or a pole, whatever) under the hull bottom at the stern, ideally following the keel line, does the ruler align nicely with the bottom side of the cavitation plate?
My guess is that the cav plate is actually a bit lower. I've seen several boats (my tender included) rigged like that from the builder, just to stay on the safe side, so to speak...

 

[MapisM]

Also some info I read last night suggested the cavitation plate should be 1" above the keel for every 12" from the transom, so at 6" from the transom your cavitation plate would need to be about 1/2" above the keel and so on.

I didn't read your last post before writing mine.
I would call that hair splitting, rather than optimization.
It is true that the notched transom (the one you can see in the first pic posted by Spi D in post #3) allows a more aggressive X-dim AOTBE, for simple geometrical reasons based on the hull AoA at speed.
But again, in a 35 knots boat with a normal transom, non stepped hull, no pad keel, no hydraulic jack plate (or any other tricks used to squeeze another couple of knots out of boats already capable of possibly achieving 70), half an inch difference is totally meaningless.