Serious Outboard Question -Size

[Lakesailor]

I have a 15ft day boat with a 70HP Johnson hung on the back. I bought the boat because it is ideal for use on the lake. The motor is a bit OTT for Windermere where the limit is 10 knots.

It's a planing hull but very stable at low speeds so would suit a smaller 4 stroke motor for pootling around the lake.

The Johnson uses about a gallon an hour at 1500rpm at which it achieves the lower, 6mph speed limit in Bowness Bay and at the ends of the lake. The 10 mph bits (knots in reality) aren't much use to me as opening the throttle to get near to 10mph the motor is producing more power than you need to achieve that displacement speed and starts to climb onto the plane so that without touching the throttle it goes from displacement speed to planing. Very difficult.
A lower-powered motor would not manage that transition (If I take a plug lead off it will hold the 10mph speed but not have enough grunt at those revs to climb on the plane - I tried that, to confirm the suspicion)

So what does the team think would provide good reliable low speed running with just enough grunt to possibly get on the plane. or at least battle a strong head wind.
Would 15hp do it? 20hp?
I don't want enough to be able to plane easily. I've got that and it's too tempting.

 

[Kawasaki]

I think a 20 hp four stroke would be the boy.
That would be enough whatever the Weather conditions and perhaps now again when You have some peeps on board and several cases of wine.
Picnic . pets etc.
Nothing against two strokes, it's just that Modern 4's are so quiet and frugal on juice.
Not that You are going to use a heck of a lot but it's nice not to have to faff about filling so often.
You could probably shove along at displacement wth a 10 but with a 20 I reckon the eng will be just humming along, no stress and with a bit left, just in case.
If thinking used however I would go for a 2 stroke.
I would think there would be more on the Market.
The 4's being a more recent addition to the Scene.
Dunno for sure about that last statement , just a thought.

 

[Kawasaki]

Another thought.
She might handle ok at displacement speeds right now.
A 15/ 20 will be considerably lighter.
Which may affect the "Pivot point" slightly.
"Bight" for manouvering will be different.
No big deal on a none Tidal Pond.
Boat will handle a bit differently at slow speeds and parking.
In fact as I type, because the motor will have less "response" as You click in and out of gear (smaller prop etc) it may be better for nudging about.
Not that You are operating in a crowded Marina.
So as I type , I'm thinking it may be easier to have a little un than a Big un.
Plus You have Moored/ Parked saily things so a mobo will be a doddle anyway whatever lump on the back.
You've just got My cogs working, that's all.
Won't Delete.
Next Please!

 

[Lakesailor]

Not many used 4s about, except Honda, and I'm not struck on them. Frankly, new seem a better deal as used outboards are pretty expensive until you get into basket case country. I'm never convinced by people who say "only done 30 hours" for a 6 year old motor. If true, it can't be good for a motor to run so little.

 

[landlockedpirate]

Years ago we borrowed a 14ft CJR (Dory type hull) with a 25hp/2 stroke. It would just about climb onto the plane with 1 on board. I think it would be a bit lighter than yours, and because of the hull plane a bit easier.

I would guess you would need about 30hp to just get on the plane.

But why bother, a decent 15 or 20 should get you very close to the limit.

 

[[2068]]

If your motor works reliably, don't change it!

Instead, how about a set of trim tabs, or something that will enable you to hold a lower speed in a kind of semi-planing mode. If my S28 can drop to 15/16kts with full tabs deployed and not fall off the plane, then I'd expect your hull to to be able to do 11/12kts similarly...

dv.

 

[Lakesailor]

Well that's a point. It'll cost me to change, so the better economy is a bit of an illusion.
I'm not rushing at this. It's the trolling speeds (not that I fish) that need to be reliable. Kawa's point about bite is good, but actually I rather like the instant stop when I put it astern. Makes stopping at jetties a doddle. The 4 hp motor on the saily boat struggled to stop it.
I don't think I'll do anything fundamental this season, but as a long term idea I rather like the idea of matching the motor to the use it gets.

 

[Kawasaki]

Yep new is obviously the way to go with outboards.
I was just thinking about the Tradition of Northern Frugality!
I too would prefer a Yam or Suz.
Had all three at one time or another in various sizes.
No doubt Honda make good products.
I was a Honda bike Dealer + the others and have been involved in Honda cars.
Would pick a Yam or Suz before the Big H in the outboard World though.

 

[[2068]]

The other thing to consider would be to fit a prop with a shorter pitch...

...depending on how the power curve matches what the hull is doing: at the moment it sounds like the torque (thrust) is increasing just beyond the point at which you start to plane. which means that once it starts to plane, it's waaheeey and off you go at much higher speed.

A shorter pitched prop (a couple of sizes down) will move this waaheeey point to be a lower speed, and make the whole thing more controllable.

dv.

 

[Kawasaki]

No ,wont work on the Lake.
At semi displacement mode, making wash and all that, not good.
The idea is to slip along at 6 ish most the time as I see it.
10 now and again.
Pottering about all the time with a 70 does not make sense anyway.
Motor doesn't like it.
OK for now and again.
OTT as our Learned Soft Porn Poster says.
Sexy bird with outboard piccie coming up?
Challenge!
Anyway the cost of Tabs and fitting, plus the Mither.
Good thinking though but No I still think a 20 is the way to go.
Soon sell the 70, good Market for used stuff that size.

 

[Lakesailor]

Yup.
Hypothetically of course. If I accelerate to about 8 or 9mph and set the throttle there it will start to climb on the plane and once there (about 14mph) it accelerates (hands off throttle) to about 25mph. All without any input from me.
But would a smaller pitch prop would need more revs to overcome the displacement drag at 10 mph. The motor would be spinning faster and have more spare torque to accelerate onto the plane. Or am I wrong on that? Would the lower thrust in the prop prevent that?

 

[hlb]

Not sure why you want to change a perfectly good engine. Theres nothing going to solve your problem. At ten knots your getting into hump mode, so the boat stands on it's arse. Requiring loads of power, which is not needed once over the hump, till reaching 20 or more knots. With a smaller engine, you may reach 8 knots, maybe not. Point being, you need more power to do 10 knots constant, than 20 knots. Now if they had made a speed limit of say 20 knot's, or eight knots, it would have been easy. Ten knots is nigh on impossible for any boat, unless it's 60ft plus. as is 11, 12 or 13. Which is the speeds you begin to plane. If you have a planing hull. So basically, no engine is going to give you 10 knots, flat out all the time. It will get stressed and over fueled, or go over the hump, then ending up at about 20 knots.

Not sure if I have explained this well. /forums/images/graemlins/confused.gif

 

[Kawasaki]

Think You are tying us up in knots! /forums/images/graemlins/tongue.gif

 

[Kawasaki]

Ok , Rewind.
Pootling is the order of the Day.
Once the stern "digs in" 10 kts ish with Lakey's hull, wash starts, even a bit less I would imagine.
We have a planing hull here.
On a Lake.
OK Said Lake can be a bit Fiesty.
When Fiesty, a bit more wake is No Prob.
So Lake Man can go a bit quicker if needs be, for whatever reason.
Cos there is already a few wavelets etc etc.
A bit more splashing about is No Big Deasl.
But a planing hull at Semi displacement speed, at 15 ft is gonna make a fair wash.
(this is not wanted I presume) at about 11/12 knots.
Is this how the boat behaves L?
Enlighten us.

 

[MapisM]

Spot on.
The situation LS describes is strictly connected to the hull type/size, not to the engine.
If the boat is frequently used at displacement speed, I'd rather consider a very small wing engine.
Most likely, 6 or 8 hp would do.
PS: you did explain it perfectly, imho!

 

[Major Catastrophe]

Another solution would be to enter the offices of the LDNPA and machine gun the entire board and place yourself as supreme ruler of Windermere, your first executive act being to remove the speed restriction. IMHO.

Prolly cheaper than a new engine.

 

[[2068]]

No it's not, it's a combination of the engine torque curve AND what the hull characteristics are.

If the torque curve fell away or flattened off shortly after the hump speed, you wouldn't get the "running away", which is where the suggestion of shorter prop pitch is going.

For similar reasons, VP engineer their big engines to have a torque curve that falls away gently at cruising speeds. Reasoning being, that if you hit a few waves, it pushes you down a few revs, and MORE torque to compensate. Flat water, a few more revs, LESS torque, to stop the engine running away (all for a given throttle position). In this respect, a 4 stroke 30-40hp might be better than the current 70hp 2-stroke, in that the torque hump might occur lower down the rev range.

dv.

 

[[2068]]

>Ten knots is nigh on impossible for any boat

Wrong! I can do 10kts in the S28, so why can't you?

It's a cr*p speed to do, though, superchargers whirring away, ar&se stuck in the water, huge wake being thrown, 40litres an hour for no progress, but it can be done.

dv.

 

[Lakesailor]

I tend to agree. At 10 knots I have to stand to see where I'm going and the motor is working much harder for a couple of knots 6-7 is much better, and easy to maintain.
I take the points about sticking with something that works, and also the ease of manouvering with prop with a big bite.
It just seems a waste of a powerful engine (and also a temptation to be naughty)
A wing engine is not an option. I want to retain remotes and steering which will get tricky with 2 motors on a small boat.
The LDNPA doomsday scenario sounds appealing, but I fear the Friends of the Lake District would send a bounty hunter.

 

[MapisM]

[ QUOTE ]
It's a cr*p speed to do, though, superchargers whirring away, ar&se stuck in the water, huge wake being thrown, 40litres an hour for no progress, but it can be done.

[/ QUOTE ]That's a weird conclusion of your sentence. Don't you think it would be more appropriate "...hence, in practice it can't be done"?
And that's exactly the behaviour connected to the hull and not to the engine (aside from the chargers part of it), which we were talking about. I don't think the "running away" effect was the major problem.