Outboard mounting height?

[wipe_out]

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.

Aligning with the keel seems to be the "standard" setup but doesn't necessarily mean it's the "best" or even the "correct" setup for any given boat.. It seems the only way to get it right is real world testing.. An article I read last night actually made a lot on sense when it said that having the cavitation plate in line with the keel assumes the water coming out from the back is going to stay at that height but it doesn't.. The boat makes a hole in the water as it moves and at the point where the transom passes a given point the water starts to refill the hole causing it to rise..

Having the cavitation plate set to low causes drag, increased fuel consumption, more strain on the motor and apparently more strain on the transom (though I am not sure I see how since it's the prop doing the pushing unless it's to do with the leverage).. They also said having it at the correct hight can improve time to plane.. On one of the other forums a guy said of something like the last 4 boats he has bought, all aligned to the keel from the shop he needed to raise all of them to get them running right, one he had to raise by 3 holes..

To get it perfect would require a "Jackplate" but it should get close enough using the mounting holes..

So yes, maybe it's OTT or could be seen as unnecessary but then can't the same be said for the amount of time and money we all spend on boats? Why not get the outboard mounted right?

 

[wipe_out]

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.

MapisM, Thanks for the comment and what you say makes a lot of sense.. The point of it all really isn't to gain any top speed, 35kn is plenty for our needs.. If anything an efficiency gain/slightly better fuel consumption would be a bonus.. For me personally it's more about getting it setup right.. I am not a fan of things not being right because you "probably won't notice", call it OCD or whatever but if I know it's not right it will bug me..

Anyway, when I am next out I will do some testing and see where the plate is at WOT and trimmed correctly..

 

[sprocker]

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

I aim to do a couple more runs, one with a full fuel tank, as my initial run was half full, just to see if it makes any noticeable difference. Then i'll probably wait until July when I lift out for a scrub, so I only have one lift out charge.

I'm also thinking about a different prop once I have the engine height sorted, I'm only pulling 5200 revs at WOT when I should be reaching 5800/6000.

 

[Spi D]

... I'm also thinking about a different prop once I have the engine height sorted, I'm only pulling 5200 revs at WOT when I should be reaching 5800/6000.

If rpm are still out of rec. range after altering the setup, you def. need to address the issue. All else equal 1" change in pitch will give you a 200 rpm change (lower pitch=higher rpm and vice versa). Better near the upper limit of range.

 

[MapisM]

I am not a fan of things not being right because you "probably won't notice", call it OCD or whatever but if I know it's not right it will bug me.

Haha, trust me, I understand that. :encouragement:
Anyway, even if you probably won't notice (!), I'd rather want to see the cav plate skimming the surface also at less than WOT - let's say at least above 20 knots or so.
If for you that will mean raising the o/b one hole, or even two, I don't think that can make things much worse with respect to cavitation in a following sea (as per your reply to AndieMac in post #15).
Cruising in a following sea on a small(ish) P boat with an outboard will never be a piece of cake, no matter how low you bury the leg into the water.

 

[sprocker]

For me personally it's more about getting it setup right.. I am not a fan of things not being right because you "probably won't notice", call it OCD or whatever but if I know it's not right it will bug me..

Exactly my sentiments! ( I blame it on being an engineer).
After all, part of the 'fun' of boat owning is fixing stuff & fine tuning......right?

 

[AndieMac]

.............Cruising in a following sea on a small(ish) P boat with an outboard will never be a piece of cake, no matter how low you bury the leg into the water.

Too true, it's such a fine line between keeping the bow from burying into the back of the wave in front with up trim and the prop venting in big short steep seas, then controlling your speed build up between waves.

 

[gordmac]

The fishing guys tend to mess about with height as they use a lot of fuel and a saving always helps!

 

[wipe_out]

For anyone who is interested I finally had a good day to try out the boat since lifting the outboard one hole.. To me it felt like it did help in getting on plane more easily and fuel consumption dropped slightly.. I wouldn't say these changes were dramatic but certainly noticeable..

Also reached a speed of 36kn quite quickly before feeling the boat actually go airborne over one of the swells (would love to have captured that on camera) and then slowed down before "someone" moaned at me..

What I would say is the wake and the spray at the back is much cleaner, no more big sprays coming up from the drive which I guess were from the cav plate being under water.. Don't have a before and after video but this is what it looks like now..

https://1drv.ms/v/s!ApmilRHUdiobgXN4vIrEGUXy5rC1

 

[MapisM]

The wake does look nice and flat, but that pretty much depends also on the trim, and it would be impossible to tell what works better based on a video anyway - even if you would have done the same video "before".
Otoh, from what you are reporting, it seems that the trick worked, therefore, well done! :encouragement:

Just curious, if you look at the o/b bottom while cruising, can you now spot the cavitation plate?
Mind, you shouldn't see it high and dry - more like "mixed" with white spray.
But it should be somewhat visible, not constantly buried in the water as you said it was.

 

[wipe_out]

Just curious, if you look at the o/b bottom while cruising, can you now spot the cavitation plate?
Mind, you shouldn't see it high and dry - more like "mixed" with white spray.
But it should be somewhat visible, not constantly buried in the water as you said it was.

I couldn't convince my "crew" to hang off the back with the camera yesterday but will do that at some point to compare to the video I took of the cav plate before raising the outboard.. That would be a good comparison of the amount of how the water flowing over the car plate has changed..

Interesting you mentioned trim, I am not sure it it's my imagination but the outboard trim seemed to be more effective as well yesterday.. When trimming out after getting on a plane the way the spray moved back on the hull felt far more pronounced and noticeable.. I would have thought the engine being higher would have reduced the effectiveness of the trim but maybe the reduced drag actually improved it..