Best prop for underpowered boat

[wmbreck]

I have a 15 foot fiberglass Larson runabout from the mid-late sixties in great condtion. Motor is in great condition BUT way to small. It was used for fishing only by the previous owner. Has a Johnson 28 SPL from about 1992 and I can't afford a new motor. Last year I put a Hydrofoil and I think it got on plane after a few minutes with just 2 passegers. My question is this. What size and pitch prop would give me the best top end as I don't mind taking a little longer for it to get up if I could get 3-4 more mph. Highest we hit on GPS in calm water was about 19-20. Boat alone is about 850 ilbs. So again, is a larger diameter prop better and higher or lower pitch for more top end. Some have said I should be able to get 25+mph even with the 28 hp if trim is right and compression good - and it is.

Thanks

 

[steelespike]

Re: Best prop for underpowered boat

We really need rpm and speed and the present prop size.A tach is the only way to know that the motor is operating within its rpm range.Check out tinytach they are reasonable and easy to install.
In the meantime check the motor height.The foil should be clear of the water at speed.
And the higher you can get the motor the less drag.I usually suggest raising the motor to a point just before venting becomes a problem.There may be some holes that allow vertical adjustment otherwise you can just raise it using a stick to hold it while you tighten the clamps.Just be sure it has a good grip.Also be sure the throttle opens all the way.Your prop size is on the barrel or under the prop nut.I don't find a 92 28 hp 89 is the latest in my info.If you get the model numbers
we can probably date it. If you read the stickies at the top of the page and the helps on the prop store page it will help you understand prop selection better.Your motor needs to operate close to 5,500 rpm at wide open throttle with a light load.

 

[wmbreck]

Re: Best prop for underpowered boat

hi, if you check this shortened link on this iboats.com site http://tinyurl.com/4owhtzx you'll see that 4 props are offered for the johnson 28spl. it was made from 86-96 and mine is about a 91-92 from what I recall. it is also a long shaft which i'm hoping does not put the foil too low in the water to create drag. it seems to need that to get it on plane which takes a few minutes of course. I have it trimmed to the best top speed I can get with the current prop which may be the wrong one as it is not the original. I hope to have the spark advance lever and throttle checked to see if they are opening fully. so, not knowing much, i assume my only shot at a few more rpms and a few more mph with this setup is the best possible prop size and most efficient pitch?

thanks again

 

[steelespike]

Re: Best prop for underpowered boat

If the transom is 20 " measured vertically the long shaft should work But the foil needs
to clear the water to make any real speed.
I believe 11" is the standard prop.but you may allready have a load prop.See if you can find the numbers.Load would likely be 10 or a 9"Diameter is a secondary consideration,Mostly to be sure it physically fits.
While this is very vague speeds predicted with heavy loads and 9" props is 10-20 mph.
So it looks like your close though I fully believe you need to tinker for more.
Weight elimination and placement should help.Are your tanks and batteries in the stern?

 

[wmbreck]

Re: Best prop for underpowered boat

Here are a few shots of the boat and motor.
http://johnsonlarson.homestead.com/index.html

You can see both props. The black one came on it and I was told he used it for fishing which I assumed wouldbe the slowest. The white one was taken of a 25hp I had and is much bigger and fatter - only about 1/2 " space `left or it would not have fit. I never ran it with the black one so I'm not sure what it would do. I put the white prop on at the same time as the hydrofoil and 'discovered' the spark advance at the same time. Yes I am that stupid. So we went from 7-10 mph to about 18+ with the 3 changes plus a slight change in the trim - outward. Still not sure everything is adjusted right because, before the spark advanced entered the picture, I was messing with everything because I knew 28hp should do better than 10 mph even on a heavy boat like this.
Pernaps the white prop is too big, fat, wrong pitch? And again, maybe the throttle is not totally right and the spark advance not pushing as far as it can. I've had some say I should get 25+? So I would be satified with getting this up to 22-23 with 2 aboard. For any more I'll need a 40-50hp. It will take a 65.

Nice riding boat though. Hull looks like a combo of a V and Whaler together. I do have a movie of it doing about 19-20 with just me in it but the file is too big to send, Thanks again for the info. can't do much now because it is at lake Ontario and I'm in Jersey, but as soon as I get up there, I'll get the props off and take it from there.

Thanks again.

 

[pootnic]

Re: Best prop for underpowered boat

Sharp looking boat.
Like mentioned above,you really need a tach.
Could you get on plane without the foil?
In all those pictures was waiting to see the one showing the plate in relation to the bottom of the boat.
Your motor might be able to be lifted abit=less drag.(again mentioned above).
If it was me and had no tach.I'd take everything out of it I could(weight),go out by myself and mess with the pin settings on the outboard,might even take the foil off(put back on) and raise the outboard to see what happens there.
Your question on prop pitch is a loaded question,with alot of varibles.
Bigger pitch should get you more speed but at the proper rpms.
You have to get on plane quicker so a smaller pitch would help but you may be over your rpm range after it gets on plane.(probably slower to)
Rpms is real important with outboards,your outboard is supposed to be at a certain rpm at WOT(wide open throttle).
To little rpm is just as bad as to much rpm and both will kill the motor over time.

 

[steelespike]

Re: Best prop for underpowered boat

I don't remember the details but we talked before.I remember the pictures.
That is the old style gearcase.that was changed to a modern through hub in 85
So the gearcase isn't a 92.I would suspect the same of the motor.
You won't be able to run it as high.

 

[wmbreck]

Re: Best prop for underpowered boat

For what it is worth, I just found a short clip of me alone driving the boat. This is after the foil was put on, the spark advanced used and the larger white prop that still may not be the right one for top end. There is very little extra weight in the boat and again just one passenger here. I'm just hoping that the throttle and advance can be give me a little more if both are not set to open to the max and that the right pitched prop may help. Also, as you say, with the long shaft motor maybe has the foil dragging a bit now and if so maybe I can raise the motor up on the tansome an inch or two with some kind of filler. The trim is where it seems to do best now which is only one slot from being all the way out? I'just like 3-4 more mph. But the question here, if you can see this, is whether or not I'm am on plane from what you can tell? Thanks.

http://larsonflyby.homestead.com/larson-fly-by.html

 

[steelespike]

Re: Best prop for underpowered boat

I can't view the video but it should be getting up on the water and the bow not plowing.
Speaking generically What they call "sync"is done by adjusting the stator cam (that opens the carb.Usually there is a line or lines on the cam where the carb roller just begins to touch as the stator advances.then the carb continues to open till the timing advances all the way.
Both carb and timing may be able to be adjusted to open more or less as required.
You may be able to put the video on youtube put the link here.
The foil should help get on plane but if in the water at speed the drage will slow you and may make it difficult to get the bow up right.

 

[wmbreck]

Re: Best prop for underpowered boat

Thanks. Sorry about the video. I just put it in internet explorer and it played fine using Apple's Real Player. Maybe yor player is different but at least I know the link is alright. Never done a youtube but I'll see if someone knows how and can help so you can see 'exactly' what I've got at the present anyway.

 

[wmbreck]

Re: Best prop for underpowered boat

Figured out youtube so here you have what it is doing at this point with just me aboard 18-20mph.
I'd like to get it 24-25 if at all possible with the few things that might be changed or not as perfected as they could be, rpms, prop, foil, throttle, spark advance, 'sync', weight etc.

http://www.youtube.com/watch?v=duumURTsQHA

 

[steelespike]

Re: Best prop for underpowered boat

I got it to work. For 18 to 20 mph the boat looks pretty good.Up on the water reacting to the waves. Motor sounds like its winding up pretty good. I forget;have you looked for prop mumbers near the base of the blades maybe under the nut.Is a Tinytach or something like it in the works?
On your model number the 2 letters next to the last are the year" EN" would be 92.

 

[wmbreck]

Re: Best prop for underpowered boat

ebay had a chart and I figured it out eventually to this below.
The no's 'seemed' to fit perfectly.

Our S/N - J28ESLEIB

J - Johnson
28 - hp
ES - electric start
L= long shaft - 20"
E I = 91 ....Year built
B - Mod. variation = Belgium

 

[steelespike]

Re: Best prop for underpowered boat

Looks like you have a 91 powerhead with a pre 85 gearcase(non through hub)
Perhaps thats the Belgium variation,useing the old style gearcase.A simple proven gearcase.It just can't be run as high up. Prop selection may be limited.If I'm reading the charts right the 28 doesn't have a through hub gearcase.
If you search the prop page for 28 you get 86-96 13 spline dr. non through hub. and 62-64 pin drive non through hub.

 

[wmbreck]

Re: Best prop for underpowered boat

Because of my limited boat knowledge, I'm not sure I follow all you are saying on the technical end. But are you saying that, unless there can be some rpm gain via the some throttle linkage movement that this is what it is? If so, I guess I have to live with it unless I stumble onto a nice 40-50 hp.
Maybe all I can do is make sure that the foil is not dragging and that the trim is at the best spot. I guess increasing the hp via carberation is out of the question financially. Hard to accept that something can't buy me 3-4 mph but maybe that's the case. I'll at least drag both of those very different props into a shop and see which is the best for high end.

Thanks

 

[steelespike]

Re: Best prop for underpowered boat

I did say in my post regarding the movie that you should still try to get more speed.
You need to be sure all is setup right.
Without going back and checking all the posts I can suggest that you be sure the motor is running as high as possible.(antiventplate just above the prop as close to even or above the bottom as the old style gearcase allows).In the extreme this could add a couple of mph.
and slightly improve hole shot.
but could require a homemade jack plate or build up the transom.Lay a stratght edge along the keel toward the antiventplate and measure where the plate is.
Check the link&sync I described you will see the stator rotate as you turn the throttle you should be able to note the carb roller making contact with the stator cam.There are marks on the cam indicating where the carb roller should first make contact.then just be sure the stator rotates fully to the stop and the carb is opening all the way.
Without an rpm reading there is no way to know if your motor is reaching rpm or if its overreving.
or if it is at the ideal rpm.
If we knew your rpm and present prop size we could tell better if and what size prop might solve your problem.

 

[gbrowne9]

Re: Best prop for underpowered boat

there was a thread a year or so ago about turning a 1985 or 86 20hp johnson into a 35hp. basicly take the carb off and see if the barral of the carb is the same size as the hole of the intake.if the intake hole is smaller take the manifold off and make it the same size.for the complete mod you would have to get a carb off a 35hp.i did this to my 1989 20hp and they,re was a increase in power.