Am I exagerating or this engine is still too low?

[babbot1]

Originally Posted by Last Tango

That is complete overkill (as is jackplates and the like) for the OP's problem and will cost five times the price!

Porpoising is the result of a fairly minor trim issue that doesn't take much to sort and I'd rather be drilling 4 X 6mm (easily re-filled) holes in the cav plate than drilling into the transom.

The Doelfins will completely eliminate the issue and they can be adjusted with the engine's trim to suit the boat speed. I've fitted fixed trim tabs to boats before and on both occasions they solved a low speed planning issue and created a ploughing problem at high speed.

As gtflash says. You might have to move the engine back down as well to sort this but I'd just get a set of Doel fins (for all they cost) and give it a try.

Just sharing my exact experience with a severe porpoising problem.
Smart tabs (not trim tabs) are in the realm of $125 US depending on the size. Seems pretty cheap to me. Only a few screws in the hull and not overkill.
It was an easy install and worked awesome to eliminate proposing. Plus increased hotshot acceleration (bonus).

[beamishken]

Originally Posted by Last Tango

That is complete overkill (as is jackplates and the like) for the OP's problem and will cost five times the price!

Porpoising is the result of a fairly minor trim issue that doesn't take much to sort and I'd rather be drilling 4 X 6mm (easily re-filled) holes in the cav plate than drilling into the transom.

The Doelfins will completely eliminate the issue and they can be adjusted with the engine's trim to suit the boat speed. I've fitted fixed trim tabs to boats before and on both occasions they solved a low speed planning issue and created a ploughing problem at high speed.

As gtflash says. You might have to move the engine back down as well to sort this but I'd just get a set of Doel fins (for all they cost) and give it a try.

I've got to agree with this, I'd say the problem is probably a short hull with a lot of weight in the stern, the doel fins will just give that little extra lift to keep the stern up and are variable with the power trim of the engine
For the sake of 40 quid I'd rather 4 little holes in the cav plate than holes in the hull

[savard02]

Lots of good points here! Thanks everyone! It is giving me plenty of things to try out.

I think that I will go with the obvious first. Moving weight toward stern and try to find an appropriate COG / bow lift equilibrium for my usage. I also need to play more with the trim and see how it react. Previously (before I raise the engine), I had to keep a negative trim (propeller closer to engine) else it would porpoise too much to be able to reach WOT. Now, at least I can trim to neutral position and even get some positive trim at WOT and the boat keep stable (and go faster / and handle better). Before you ask, the transom is 21inch, so no I don't have a long shaft on short transom .

As pointed out be someone here, this boat is quite small (hull is 3.5m long, even if the overall boat length is 4.5m) so it might not take much to get it out of balance.

I tought about doelfin and smart trimtab. Smart trims are not very practical for me because I don't have much spaces on the transom to fit them. I have an electric trolling motor taking all the space on one side and depth finder on the other. So I would prefer going with the doelfin if needed.

I haven't completely ruled out the set-back of the engine. It seems to be pretty common for this manufacturer to fit a "hull extension". It is offered as an option. There's pictures on their web site: AIRSOLID-15 . The red boat has one fitted. It's an aluminum hull but fiberglass was offered with it too at the time I bought mine.

Finally, looking underside of the hull, on the planing pad I can see that it's slightly crooked about 0.7m from the transom (going toward bow) longitudinally to the direction of the boat. To speak clearly, it's making the "banana".
I don't know how much of a difference it can make, if it was done intentionally or if it's a manufacturing error. I will try to post a picture later tonight.

[gtflash]

Banana shaped convex in direction is called hook if I t was designed into the hull, it is Put there to make it run nose down by lifting stern. Of course it could be a mistake or caused by a bad trailer.

Armed with that info and the fact you trimmed in to eliminate porpoise I reckon weight forward might be the answer. Never know to you try it. Builders sand in 20kg bags is pretty cheap and if the worse comes to the worse chuck it overboard. My first boat was 6m long with inboard and 20kg changed its whole attitude. Good luck. Keep us posted and get some more pics. Film cav plate and don’t be scared to play.

Cnc jack plates sell 2”aluminium engine spacers. They are very cheap, if weight aft proves to cure the ride. Maybe invest in some of them.

[eze]

Hi there Sav,


Just a quick question here,


When you run the boat with family on the bench seat, do you operate the tiller from the bench seat also or from aft in the stern?


Do you still suffer from porpoising with family on the bench seat?

[savard02]

Originally Posted by eze

Hi there Sav,


Just a quick question here,


When you run the boat with family on the bench seat, do you operate the tiller from the bench seat also or from aft in the stern?


Do you still suffer from porpoising with family on the bench seat?

I operate the boat either standing behind the bench or seated on the float.

My estimation is that center of gravity with me alone is about 30cm stern of the fuel tank. Which is also about where the hook in the hull is also located.

Previously with the family and the engine fully trimmed in it would not porpoise. But would start porpoising if trimming out. Haven’t tried yet with engine raised.

[Last Tango]

Originally Posted by savard02

I operate the boat either standing behind the bench or seated on the float.

My estimation is that center of gravity with me alone is about 30cm stern of the fuel tank. Which is also about where the hook in the hull is also located.

Previously with the family and the engine fully trimmed in it would not porpoise. But would start porpoising if trimming out. Haven’t tried yet with engine raised.

Attachment 122491

There's obviously a bit of distortion from the wide angle lens but the layout looks "bow heavy" and it looks like that's an extended tiller meaning, you're probably not sitting where we might have been thinking either?

[savard02]

Originally Posted by Last Tango

There's obviously a bit of distortion from the wide angle lens but the layout looks "bow heavy" and it looks like that's an extended tiller meaning, you're probably not sitting where we might have been thinking either?

I went for a ride today with my 5yo boy! So I had the chance to take a few more pictures.

It’s a regular etec tiller which are pretty long
I installed the the 4” setback + moved all the weight closest to the transom ( fuel tank, battery, me and the boy). I had a hard time getting on plane. Bow rising probably >30degree and engine stuffing. But once on plane, with me literally seated over the transom, it ran pretty good at all speed with neutral trim. I tried to gradually moved the weight aft, but to no success.

My last option will be to give a try to the doelfin... because I don’t really fancy putting 150kg of sand bag in the bow!

[Bigplumbs]

If you do need ballast use water not sand. Are you sure you mean 150 kg

[savard02]

Originally Posted by Bigplumbs

If you do need ballast use water not sand. Are you sure you mean 150 kg

150kg ==> 2 guys in the front. Then it runs fine... The problem is when i'm alone.

[savard02]

Ok, after two weeks of deliberation, raising and lowering the engine, moving the weight from the front to the back, the judges finally handed down their verdict.

And the big winner is: doelfin!

At 3 o’clock this afternoon a bubble popped in my brain (I guest it’s caused by the idea of not ribbing/fishing for the next 6 months of winter) I ran to the store and bought the doelfin. Installed it, and ran to the river to test it. I couldn’t believe it but it literally changed the boat. It was raining, it was freezing cold but I was smiling from ear to ear!

It jump on plane in 2 seconds, keep stable at all planing speed and it didn’t even changed the top speed.

Really glad that it was sorted out before winter! Now, I just couldn’t wait for the next season.

Some notes: this hull seems to be a bow heavy runner. Since I’m navigating only inland water I will have to live with it. The doelfin will fix the problem for when i’m alone. I’ll see how it will go with more people in the front but it doesn’t worry me. I could have also opted for smart tab but they were not practical because of my setup and more expensive.

Thanks everyone for your great comments! I owe you all a beer if you ever pass by Montreal! Just pm me :-)

And just for bragging here’s a video of me going back to the boat launch on the beautiful Mille-Iles river. (quality is bad again, but the idea is there:-) I need to ask Santa for a gopro this Christmas!) : https://youtu.be/O_A0TuWWYDg

[Bigplumbs]

Excellent news. I am very pleased for you. Effective and quite a cheap fix. Well done


Dennis

[jeffstevens763@g]

good news, to help others a few tech points height of engine, trim position, WOT rpm, gps speed, prop dia & pitch, weight all up.
glad you got sorted

[eze]

Nice one Sav,
glad you achieved a positive result...

[savard02]

Originally Posted by jeffstevens763@g

good news, to help others a few tech points height of engine, trim position, WOT rpm, gps speed, prop dia & pitch, weight all up.
glad you got sorted

Good point!

Height of engine: ~3/4" (~2cm) over the bottom of the hull. it's high but it didn't ventilate and allow the doelfin to be over the water most of the time when on plane.

Prop: 15" pitch x 13.5" dia, solas amita 3, 3 blades

WOT rpm: 5200 RPM, too slow, will give a try to my 13" pitch ss prop to increase RPM.

gps speed @ WOT: 49 km/h (30 mph), I was able to keep the boat on plane and very flat at 24 km/h (15mph). I could probably have gone even slower. More test to be done next time I go on the water.

weight: lightly loaded, just me (85kg) and about 20l of fuel (+ 7kg anchor, bench, battery, etc.), The weight seemed to be distributed slightly toward stern. But the balance wasn't as important as previously.

Let me know if I forgot something!

[jeffstevens763@g]

If you get chance before the freeze would be nice to see how the 13 inch pitch does as a comparison

[savard02]

Originally Posted by jeffstevens763@g

If you get chance before the freeze would be nice to see how the 13 inch pitch does as a comparison

I would really like to do it but it’s going to be difficult. Insurance stop covering the boat on november 15th and we had heavy rains last week. The river is at record high level for this period of the year. It want from 100 cubic meters per second to 640 m3/s in 4 days.


See the picture of the dock which is about 30 foot long and was in about 2 1/2 foot of water last week!:

[eze]

Dam.....who to blame.....Donald Trump or global warming?