Am I exagerating or this engine is still too low?

[savard02]

hey guys, first I have to say that I really appreciate the discussions and opinions on this forum. I've been a long time reader, not so much of a contributor, but I learned a bunch on ribbing from you.

A couple of weeks ago I decide to bite the bullet and lift up my engine to the fourth hole (highest setting). I did it in order to improve the general handling, to increase wot speed and most importantly to reduce porpoising at high speed (mostly when I'm alone in the boat)!

After a run on the river I can say that the behavior at WOT has improved a lot even if, in my own opinion, there is still a lot of water splashing over the anti-ventilation plate.

The problem is now that at cruise speed the boat purpoise a lot more than before. I tried different weight distribution (more weight at the front or at the back) but to no success.

Tech Spec:
4.5m see picture below...
40 hp Evinrude Etec (tiller)
15" pitch propeller (solas amita 3?)
cruise: 3500 RPM, 31 km/h
wot: 5460 RPM, 49 km/h

Sorry for the poor quality of the video, I was alone. Also I shouldn't have enabled the auto-stabilisation of the image in youtube as the engine looks like it's moving all around like it wasn't correctly fixed to the transom but that's just visual artefact of the stabilization...
I've put the entire video here: WOT / correctly trimmed is @ approx 1:15

https://youtu.be/MupKH9528iU

Here's a picture of the engine height in relation to the boat:

As I can't increase the height of the engine anymore, I was thinking about buying a fix set back/ jack plate like this one: Hi-Jacker™ Fixed Jack Plate 4 inch | T-H Marine Supplies

It would allow me to increase the engine even more and maybe get the engine running in "cleaner" water because of the 4" set back.

[beerbelly]

wouldn't go any higher than that also is it just the pic but your outboard doesent look as if its tilted all the way down

[charliee]

Plus 1 on not down.

[two stroke mick]

I would check the specs for wot for that engine as it would appear to me that you are not within the specs for the i2 etec. I think you should be achiving circa 5750 rpm at wot. I do agree that the engine height looks right. Tsm

[Last Tango]

You've not got a lot of weight up near the bow?

The porpoising starts when the leading edge of the whetted surface falls behind the centre of gravity of the boat and it's relying solely on the prop thrust to keep the bow in the air. Moving weight aft can help. (and check the engine's trimming down all the way)

[savard02]

I’ll post a new picture tonight with engine trim all the way down. engine is trim to ‘neutral’ position On previous picture.

For the weight, I guess you’re right ‘last tango’. I tried to move as much weight as possible towards the front. but still it’s a tiller boat so I have to seat in the back. 12g Fuel, batteries and seat in the middle (a bit towards bow). Anchor and safety kit in the bow locker.
I guess I will have to bring someone with me to seat in the front to counter balance my weight! :-)

[Last Tango]

Originally Posted by savard02

I’ll post a new picture tonight with engine trim all the way down. engine is trim to ‘neutral’ position On previous picture.

For the weight, I guess you’re right ‘last tango’. I tried to move as much weight as possible towards the front. but still it’s a tiller boat so I have to seat in the back. 12g Fuel, batteries and seat in the middle (a bit towards bow). Anchor and safety kit in the bow locker.
I guess I will have to bring someone with me to seat in the front to counter balance my weight! :-)

Sorry.
I meant to move weight back to stop porpoising. You want to get the centre of gravity behind the leading edge of the whetted area.
Too much weight forward will cause the problem

[smallribber]

Originally Posted by Last Tango

Sorry.
I meant to move weight back to stop porpoising. You want to get the centre of gravity behind the leading edge of the whetted area.
Too much weight forward will cause the problem

You originally said Aft which is back. Did you mean to say forward ?

[Last Tango]

Originally Posted by Bigplumbs

You originally said Aft which is back. Did you mean to say forward ?

No I meant "aft". I think the OP misunderstood.

[smallribber]

Originally Posted by Last Tango

No I meant "aft". I think the OP misunderstood.

But to help porpoising I think you need to move weight forward surely

[charliee]

To help reduce Porpoising you need to shift weight back toward the stern.

[Last Tango]

Originally Posted by Bigplumbs

But to help porpoising I think you need to move weight forward surely

No.
You're not trying to hold the bow down, the intent is to stop it "falling" cyclically.

Mechanics of "porpoising"

As the speed increases the wet surface of the boat moves progressively further aft until there's insufficient "lift" to hold the bow up, so it drops back into the water, produces "lift" and the cycle just keeps repeating.
Moving weight aft pushes the centre of gravity back and reduces the lift required to keep the bow up.
It's a matter of equilibrium, technically you can get away with the centre of gravity still being forward of the wet area of the hull but you're relying engine thrust to hold it there and the scenario is a bit unstable and getting into the realms of race boats.

[Pikey Dave]

Originally Posted by Last Tango

No.
You're not trying to hold the bow down, the intent is to stop it "falling" cyclically.

Mechanics of "porpoising"

As the speed increases the wet surface of the boat moves progressively further aft until there's insufficient "lift" to hold the bow up, so it drops back into the water, produces "lift" and the cycle just keeps repeating.
Moving weight aft pushes the centre of gravity back and reduces the lift required to keep the bow up.
It's a matter of equilibrium, technically you can get away with the centre of gravity still being forward of the wet area of the hull but you're relying engine thrust to hold it there and the scenario is a bit unstable and getting into the realms of race boats.

Well I never knew that!! Every day's a school day. I see the logic, but if you had more weight upfront, surely it would keep the bow down & keep the wet surface forward, stopping the bow from rising would keep the fulcrum forward, maybe??

[Last Tango]

Originally Posted by Pikey Dave

Well I never knew that!! Every day's a school day. I see the logic, but if you had more weight upfront, surely it would keep the bow down & keep the wet surface forward, stopping the bow from rising would keep the fulcrum forward, maybe??

Yeh, but it'll also force the speed down, so you apply more power to compensate...and the bow starts to lift.....etc. So you end up back in the same situation just using more power to get there

The problem isn't the bow rising. That's kind of what you want to happen to minimise the "wet surface" (it's 800 times harder to push something through the water than it is to push it through air), it's the bow falling.
Think of the boat as a "see-saw" with the leading edge of the water contact area being the fulcrum.

[savard02]

Very interesting! It makes a lot of sense. The boat is probably purposing at slower speed because there is too much weight towards the bow. Gaining enough speed then results in a new point of equilibrium lead by the thrust of the engine maintaining the bow up.

So here’s what I was thinking to do. Move more weight aft to move the center of gravity behind the wetted surface line and then mitigating the purposing. It will work as long as the engine does not go to low under the water creating more draft, resulting in less thrust / increasing instability on the wetted surface.
I was also thinking of buying a fixed jack plate w/ 4” set back which should help moving the center of gravity toward aft. Doing so will also help adjusting the height of the engine in relation to he weight of the boat.

My feeling is that the design of the hull on this boat has optimized lift to reduce the wetted surface and then increase speed / or allow smaller engine size to the detriment of overall stability.

[savard02]

[Last Tango]

Originally Posted by savard02

Very interesting! It makes a lot of sense. The boat is probably purposing at slower speed because there is too much weight towards the bow. Gaining enough speed then results in a new point of equilibrium lead by the thrust of the engine maintaining the bow up.

So here’s what I was thinking to do. Move more weight aft to move the center of gravity behind the wetted surface line and then mitigating the purposing. It will work as long as the engine does not go to low under the water creating more draft, resulting in less thrust / increasing instability on the wetted surface.
I was also thinking of buying a fixed jack plate w/ 4” set back which should help moving the center of gravity toward aft. Doing so will also help adjusting the height of the engine in relation to he weight of the boat.

My feeling is that the design of the hull on this boat has optimized lift to reduce the wetted surface and then increase speed / or allow smaller engine size to the detriment of overall stability.

I wouldn't do anything quite as drastic as that yet. Just try rearranging the loading a bit to get more weight aft and that will probably solve it.
It may not need as much as you think.

[savard02]

Originally Posted by Last Tango

Yeh, but it'll also force the speed down, so you apply more power to compensate...and the bow starts to lift.....etc. So you end up back in the same situation just using more power to get there

The problem isn't the bow rising. That's kind of what you want to happen to minimise the "wet surface" (it's 800 times harder to push something through the water than it is to push it through air), it's the bow falling.
Think of the boat as a "see-saw" with the leading edge of the water contact area being the fulcrum.

That’s exactly what happens when I put the wife and the three kids in the bow; the boat become very stable and not purposing at all! So that’s why I was lead to think that more weight in the front would solve my purposing issue when I’m alone!

But it makes a whole lot more sense to get it stable doing the other way around. What a revelation! I’m tempted to take the day off to try it out before the river freeze :-)

[charliee]

drop the engine back down so the plate on the outboard is level with the base of the hull and shift some weight aft.

[jeffstevens763@g]

I ve always gone for equal balance throughout the boat which ever boat I own never had any problems I stick with the cav plate level with the keel and trim up to suit and my boats run level. I think your engine is too high as I see it and you need to put weight up front if needed instead of people where it's the roughest ride. All in my opinion of course