confused about hydrofoil performance

[wmbreck]

I have been reading endlessly on this topic and can't seem to come to a conclusion.

Bottom line is that I have an older 15' fiberglass runabount that is underpowered until a new motor is purchased. Has a Johnson 28 SPL Long shaft that peaks at about 10-11 mph with 2 people and maybe 18 with one. I have a new hydro unit in my shed but have read from some that it decreases top end and from others in increases theirs 2-5 mph which I need badly. I may change props, but based on what I have, does anyone know what I could expect on top end along with the typical better hole shot and handling?

Thanks?

 

[ondarvr]

Re: confused about hydrofoil performance

If you read enough threads about foils (too many) then you would see they may help, but tabs and a prop would probably work better for what you need.

The reduced top speed with a foil is from poor set up, or the boat not being able to use one. It results from the foil dragging it the water at speed, the correct setup is to raise the motor until the foil is not in the water when on plane, this may or may not be possible with all boat and motor combinations.


Since you have one already give it a try, if it doesn't help just take it off.

 

[wmbreck]

Re: confused about hydrofoil performance

Hi Admiral.
2 quick stupid questions.
1.What are the 'tabs' you mention?

2. And would a long shaft mean that it 'may' be harder to get the hydro out of the water on plane because everthing sits lower to begin with?

Bill

PS I was told thew original ower changed props for fishing, so it might be suffering from that too.

 

[steelespike]

Re: confused about hydrofoil performance

There are short shaft 15" motors and long shaft 20" motors.
Your long shaft motor means it is about 20" from the bracket that rests on the top of the transom to the antivent plate just above the prop.Your transom should be about the same demension measured vertically not along the angle of the transom.
The anti vent plate should be about even with the bottom of the boat.Usually ends up about an inch above.It should be visable at speed.
The "Tabs". are attached to te stern even with the bottom and can be adjusted
like those on the tail wing of an air plane.Moved downward they make the boat plane quicker and lay down flatter.There are passive tabs like Smart Tabs and active
power tabs)like Bennets(?). Your motor should be able to run about 31 on a light reasonably fast boat.Assuming you boat is sound and hasn't absorbed water you should be able to make 20 to 25 mph even with 2 aboard.
Is it well tuned? Does the stator advance as it should?Is it running on both cylinders?Don't assume because it seems smooth and revs in neutral that its on 2 cylinders.Carefully pull one plug wire at a time and note the reaction.What size prop?Should be on the barrel or under the prop nut.
The foil will not help top speed it will more likely to lower it.

 

[wmbreck]

Re: confused about hydrofoil performance

Thanks for the info. The boat is at Lake Ontario and I'm in Jersey so I won't be able to check some of these things for about 3 weeks when I return. I may look for the "Tabs" now so I'd have them with me to try. Any idea where or which would be best to get? Not sure about buying a prop ahead of time but I'm sure that may help some. According to what I can gather Johnson made this 28SPL in 1995 and 1995 and the props may not attach the same way in one list I saw. I may have to pull it before I replace it? But the speed you mentioned 20-25 with two would be great - for a while. But the present 10-11 mph doesn't even cool me down on a hot day

Thanks again.

 

[ondarvr]

Re: confused about hydrofoil performance

The performance depends a great deal on the hull design, some 15' F/G boats are rated at 80 or 90 hp, these models wouldn't even come close to planing with a 28. Others may not even be rated as high as 28. So without a little more info it would be very difficult to say what type of performance you should expect, or what you can try that may help.

 

[wmbreck]

Re: confused about hydrofoil performance

I see what you are saying. Our max hp is 75. The boat 'looks' like it is planing (as in sitting flat when moving at full throtlle) but does that mean it is really planing in reality? If so, it is what it is, unless compression or something like that is off as you said. I may have few options other than a better prop.

I have a spare 25 Johnson tiller in the shed with some issues. If the props are interchangable I could do that and see if the 28's prop really was down graded by the former owner for fishing purposes. I put the 25 on our 13" aluminum Duranautic metal rowboat last year for fun (with 3 adults) and it almost flew out of the water - scary fast. Could not even open it up. That's why I thought this 28 thing would do a little better that 10 mph despite knowing weight kills speed like nothing else.

Thanks once again. A lot to learn and nothing that can't be addressed by 4-5 grand

 

[JimS123]

Re: confused about hydrofoil performance

Obviously. we don't know how heavy your boat is, so we can't predict how fast it should go. But, if your motor is a 28 and the boat is rated for 75, it is truly underpowered and no add-ons will help. "Maybe" a different pitch prop may gain you a very little, but not much.

Hydrofoils are a fantastic product, but they won't help you because you are looking for more speed, not reduced bow rise. Also, fins will probably cause you slower speed becausee you may not have enough power to get them to retract.

Save your pennies and get a 60.

 

[steelespike]

Re: confused about hydrofoil performance

Years ago we had a 18ft al utility 375 pounds,with a 1960 25 hp tiller Gale long shaft,"standard" prop.
100 lbs,6 to 12 gal gas oars anchor etc gear 100 lbs,6 adults avg about 140 lbs.
about 1300 lbs.total.It planed easily and probably ran about 20 mph we took many long rides.We knew nothing about prop selection or motor setup at that time so the performance probably could have been better.We4 pulled skiers with a 100lb or 125lb adult spotter.
You have the disadvantage of a stubby boat of about 500 to 700lbs but your only looking to carry 2 people.If your going 18 then its planing probably not much,
at 10 mph your not planing. Hard to believe one person carefully placed in the boat would take you off plane.Sure your allways going to be under powered
but careful setup and a well tuned motor you'll get by.I think that you can see your 28 on a 16 or 18 ft utility could do a great job.

 

[wmbreck]

Re: confused about hydrofoil performance

Thanks. I'm propably looking at about 1500 pounds with 2 people and gear so I would like to see at least 15 mph and over 20 with one person - but that is not happening now. Maybe the prop? or as someone said, not full compression or hull design. It 's a 60's Larson All-American about 14 1/2'.
It does plane with one person, but maybe not really with 2 or more even though it 'looks' like it. Is it possible to look 'flat' in th water at 10-11mph and not really be planing? Or if it is flat in the water, it is planing - but just slowly?

 

[steelespike]

Re: confused about hydrofoil performance

If you were planing the water would break aeway from the back of the boat and you would have some sort of rooster tail perhaps more like a rooster tail nub.
Have you checked any of the things I suggested?

 

[ondarvr]

Re: confused about hydrofoil performance

At 1,500 lbs you may not do much better than what you have now. I had a 15' with a 50hp, it would do 26 at WOT with one person, the other day I was in a friends boat, 15.5' with Honda 50, it did 23mph with two of us. Cut 20 HP off that with a similar hull and I can see it not planing well.

Aluminum boats in this size range can get by with much less HP, frequently the hull design helps them too.

Now recently I did put a 50 on my 19' center console, I didn't have anything close to what its rated for (150hp) and needed to go fishing. I could only get it to plow just like you described, but if I surfed down a large wave or wake I could get it to plane and speed up considerably. It would continue on plane until I backed off the throttle even the slightest bit, as soon as I did it would go back to plowing.

Do you have a pic of the boat so we can see the design.

 

[wmbreck]

Re: confused about hydrofoil performance

Here is the exact boat from the original ads in black and white. A real classic -but I do like it the old look. Everything on it works.
It is either the 142 or 152 model.

http://www.larsonboats.com/assets/pdf/archives/1965.pdf

Here are some live shots at a site I just made.

http://johnsonlarson.homestead.com/index.html

Wondering if the prop is right because someone did mention that it was switched out for one for fishing. Might be small part of the top end problem?

Thanks.

 

[ondarvr]

Re: confused about hydrofoil performance

That's a fairly big boat for a 28.

You can try a different prop (less pitch), but I don't think your going to achieve the results you desire.

 

[npauls]

Re: confused about hydrofoil performance

Keep messing around with it and try different hole settings on the transom and things like that.

I have a 99 alumacraft magnum 165cs fishing boat with a 40hp on it and everyone told me with all my tourney gear in it that I would be lucky to see 20mph by myself. With a bit of fiddling and trying different things I am now up to 28mph by myself fully packed with gear and about 26.5 with 2 people in the boat. I ended up taking my hydrofoil off to see how the boat would work and it help out alot by taking it off. I could plane faster, at a lower speed, and also get a higher top end speed. I have been messing around with all kinds of stuff with my boat since ice off and have hit a point where I am very happy with my underpowered boat. When I can afford it I will be maxing out the hp but for now this set up will do.

Try moving the motor up or down in the mounting setting.

Try some different props.

Try the hydrofoil.

Maybe even try a manual jack plate.

Move weight around in the boat and see how that helps.

I installed a manual jack plate and that was the best thing I could have done with my boat. It has helped out a ton.

I had my boat out tonight and decided to take some gear out and take the big bow mount trolling motor off of the front. When I did this it took longer to plain out so I had my fishing partner move to sitting on the deck instead of the seat and it plained out alot faster. Now I know that the motor on the front helps out alot also.

Just take it out and change one thing at a time and test it. Once you find something that works leave it and try something else until you have it dialed in as much as you thing you can get it.

Nate

 

[steelespike]

Re: confused about hydrofoil performance

A nice boat that looks like it could get up easier than some.Is it a hard chine or rounded,
If you were at wot in the pic. your really plowing.You might be able to trim out a notch. Also if thats wot you very well might be on one cylinder.