Low speed planning performance.

[BigE]

Hi guys

Looking at repowering a signature 1750l something in the 115 - 150 hp range. I would like to get the boat to hold on the plane at a lot lower speed than it does at the moment, somewhere in the 20kph range would be nice. looking for a comfortable ride without hammering the bejeuses out of everything. (mainly my back).

So what I'm asking is how does motor choice affect the low speed planning peformance of this type of hull and what should I be looking for to improve the boats performance in this area. How do props foils engine weight and tabs affect low speed planning performance.

Please go easy on the 4S Vs 2S as i real have no preferance on engine type I'm more concerned about ride comfort.


Thanks
Big E

[Chimo]

Hi Big E

From experience with a number of boats over the years foil and tabs each add positively to improving low speed planning.

If you buy tabs Lenco or Bennett with a flat plate as the tab will help without adding to drag especially when they are fully up ie not engaged.

The lighter the motor for a given HP, the easier it is to get the boat's bum up.


Cheers
Chimo

[Spaniard_King]

Go for the biggest CC's for the HP Intended which will produce the highest amount of low end torque and prop it high into the rpm range this will ensure the engine will perform down low as well.

[BigE]

Thanks Chimo & Spaniard King.

Is there any referance for which outboards make the most low down torque, or any where you can obtain outboard torque figures? I don't think i have ever seen them.

BigE

[TheRealAndy]

Planing ability is more about hull design and trim than engine performance. You cant change the planing speed of the hull. A flat hull will plane easy than a deep V, but the ride is going to be crap. Then engine might help you get on the plane quicker, thats about it.

[BigE]

Thanks Andy.
I'm not trying to turn a pig into silk purse, or reinvent the wheel, But i am trying to max the low speed planning performance as much as possible, as with all things I'm sure there are things that will help and things that will hinder the boats performance in this area.

BigE

[Smithy]

If there is such a thing as "usable torque" 4 strokes have it and 2 bangers don't. The Etec advertising rabbits on about how much torque they produce but in the real world a 4 stroke will keep you on the plane at whatever rpm you want, especially if you have a foil on it and the 2 stroke will want to jump up and race away on you once it gets the hull planing. Same downsea, the revs will go up and down a lot more on each wave or swell with a 2 and may only change 300rpm or so with a 4 stroke. My motor for instance I am usually running at 4,200 rpm to keep it up on clean plane and a big swell can knock me back to 3,900rpm but I will still keep on the plane without touching the throttle then I might race away to 4,200 on the down face of the swell but it won't race away like a 2 stroke would have.

[Spaniard_King]

Andy,

low and mid range torque is what outboard manufacturers are all after, it plays a big part in low speed planning. The more torque down low an engine has will hold a boat on the plane at as lower speed. As Smithy has suggested the 2 stroke tends to fall off the plane earlier than the 2 stroke.

[marty666]

add a foil to the outboard with tabs and more blades on the prop to help lift the bum too if you are not looking for top end possibly going to a 5 blade might be some thing to look at.

[Crocodile]

Hello BigE,

I used to have a foil but now I have wedges between the outboard and the transom, this allows the motor to really trim under a lot more.
I have measured with the GPS that the boat will now plane at lower speeds than with the foil.

On the subject of 2s versus 4s I have just bought a new motor and went for a 2s partly because the 4strokes that I drove were awful things that had no low-speed power at all and simply fell off the plane.
I can stay comfortably on the plane at 3000 RPM for 12 knots(GPS).

If low speed planing is you thing go for the lightest 2s motor that you can find.

[Captain Incredible]

Croc is on the money for staying on the plane. Under tuck is real good but be careful going downhill.

Also when propping go for a smaller pitch but larger diameter prop which will deliver more thrust down low. Check out SOLAS PROPELLORS and talk to Steve as delivery of the "torque" through the prop is essential.

The real issue as I see it is your bad back. Your hull is not a bad thing but not particularly bouyant aft (please don't take this as a personal insult) so there is going to be a problem keeping it on the plane with anything on the back.

How about a bit of divergent thinking, keep your current engine and buy a hydraulic seat? After having a hip replacement I bought a "Marine" hydraulic seat for my centre cab and now only drive seated. If it could cook I would marry it. I can tolerate far higher speeds than I could before. Instead of taking the bang out of the boat I took the bang off my hip.

Just a thought.

[swof63]

If you want real dyno power and torque figures, you can get them from the Californian air research board somewhere here http://www.arb.ca.gov/msprog/offroad.../recmarine.htm

I have noticed tabs make a big difference. I have Lenco tabs and tabs undeployed will hold plane down to 16-17 kts, tabs deployed planes down to 12 kts easy.

Cheers
Craig

[bustaonenut]

Also when propping go for a smaller pitch but larger diameter prop which will deliver more thrust down low. Check out SOLAS PROPELLORS and talk to Steve as delivery of the "torque" through the prop is essential.

Tried that on a 6m allison angler with 200hp 2 stroke. Turned out to be a real dog dieing up a big swell then running away again, was always on and off the throttle. The prop was a 17p and i think around 15.5" diameter. Installing a permatrim helped but on a lighter load, once the boat was heavy would have the same problem.

After a bit of reading we tried out a 4 blade prop and that is what is still on the boat, best thing ever can sit on the plane at 2800rpm and rarely have to touch the throttle, the motor even sounds differant like there is no load on it,

Fuel went from about 140-150ltr for 70nm trip to 130ltrs

[johncar]

Hi guys

Looking at repowering a signature 1750l something in the 115 - 150 hp range. I would like to get the boat to hold on the plane at a lot lower speed than it does at the moment, somewhere in the 20kph range would be nice. looking for a comfortable ride without hammering the bejeuses out of everything. (mainly my back).

So what I'm asking is how does motor choice affect the low speed planning peformance of this type of hull and what should I be looking for to improve the boats performance in this area. How do props foils engine weight and tabs affect low speed planning performance.

Please go easy on the 4S Vs 2S as i real have no preferance on engine type I'm more concerned about ride comfort.


Thanks
Big E

Well all the things you mention will help,
Hull design is a big factor but you can help it by:

Minimising the engine weight vs high mid range torque curve at 3000 - 3500

Tabs are an advantage on any boat in this range.

A foil could be helpful but I would try everything else first. I just say that because it not good to add stress to your engines cav plate and mountings unnecessarliy if everything else is wrong and fighting against it.

4 blade prop at a lower pitch rather than high should help. Your engine should be able to easily reach full RPM.

Well balanced weight distribution and minimise the overall load as much as practical.

It does amaze me how many buyers and boat testers, dealers focus on how fast a boat can go in sloppy conditions when for most of us it really is more important how slow it can comfortably go.

The last boat I bought I insisted on a water test and yes it was a good day for it being 20kn plus conditions. The sales rep took it out and straight up to 5000 or more RPM and we were flying across the waves with the occasional bang but not bad. He was surprised when I took the wheel and said "that's all good but I want to know how it handles at 3000RPM".
Happy to say it went very well and was still planing nicely at 12kn and under 3000RPM and I was giving it all sorts of manouvers in all directions and trim to try and trip it up but no worries.
Only after that I tried a bit of higher speed stuff which it handled just fine but it's not often you can go fast offshore so it's not a priority for me.

good luck with it.

[Noelm]

OK, as someone who has had a thousand debates/arguments on whether 2 strokes are better than 4 strokes (and I have argued on each side) I have had the same boat with standard carby 2 strokes, HPDI 2 strokes and 4 strokes, for smooth "power delivery" the 4 stroke has it all over a 2 stroke, the 2 stroke is a motor that loves RPM and will be a slug at low RPM and will (as mentioned) at times bog down, and require a hand full of throttle, then get up and going and need the throttle pulled back, a 4 stroke does not suffer this "feature" anywhere near a 2 stroke, for what you want, a suitable HP 4 stroke will be your best bet.