Yamaha 15HP (15D) Prop question

[DennisZA]

Hi all

Apologies if I am rehashing something that has already been discussed. A Google search just brings up ads.

Boat is a 4 meter fiber glass V hull. Admittedly the motor is small for this size boat but finances and red-tape are an issue as soon as you go over 15HP here.

We bought a used Yamaha 15HP (mid 80's model) 15D. Motor just needed a clean and some new plugs and is running beautifully. The only thing we didn't repair was the prop and it was really beat-up and well worn. We did a few fishing trips with the prop as it was and then I had it repaired. Its a 10 and a half inch prop.

Since the repair, with the boat fully loaded as per a normal bassing trip we have lost 4,5km (GPS) speed. We have no tach and I am guessing the only way to get that speed back is to perhaps go to a lower " prop or to have the blades made smaller? Without my mate in the boat but will all the kit the motor was definitely going a lot better. Is my assumption correct? We are not particularly worried about take off speed but prefer more top-end speed.

Any tips would be appreciated.

Thanks

Dennis

 

[steelespike]

Re: Yamaha 15HP (15D) Prop question

Welcome, If you read the stickys at the top of the prop forum It should help you understand what we need to help you select a prop.
It would be much better to have a tach Check out the tinytach or similar induction tach.
Be sure to tell them it is going on an outboard.
We need Prop pitch,Is it 10 1/2" pitch? and lightly loaded wot rpm and gps speed. We may be able to do without rpm initially.
Your boats dry weight will help as well.We may be able to find weight with the brand and model info.
Your prop pitch and lightly loaded wot speed will get us started.
There appears to be 3 distinct type props from the early 80s to 2009.
Non thru hub pin drive,76-83,
Thru hub 8 spline 84-2009.
76-83 is listed for 15A, 84-2009 is simply listed for 15.
The other is apparently a bit obscure listing only an 11" pitch.

 

[DennisZA]

Re: Yamaha 15HP (15D) Prop question

Hi Steelespike

Its an Impala speedboat hull, circa 1950 odd.

I will definitely see if I can find a tach.

It weights around 380kg with 2x batteries, motor, tank and trolling motor.

Yes the prop is a 9 3/4 10 1/2" (that's what it reads on the stamp) Thru-hub.

With just me on the boat (with above equipment) it levels out at 21km p/h, with a mate and same equipment 13km p/h. 85-90kg guys.

Prior to the prop refurb, 16km was average with us and the above equip.

 

[limitout]

Re: Yamaha 15HP (15D) Prop question

Hi Steelespike

Its an Impala speedboat hull, circa 1950 odd.

I will definitely see if I can find a tach.

It weights around 380kg with 2x batteries, motor, tank and trolling motor.

Yes the prop is a 9 3/4 10 1/2" (that's what it reads on the stamp) Thru-hub.

With just me on the boat (with above equipment) it levels out at 21km p/h, with a mate and same equipment 13km p/h. 85-90kg guys.

Prior to the prop refurb, 16km was average with us and the above equip.

I think when they refurbished the prop they must have changed the cupping or removed too much material and the prop is too far from its stamped specs now.

this is why I am not a fan of refurbing props on small motors, that plus the cost of doing it is often near what a new prop would cost you.

refurbing props is for when you cant find replacements or if the props are expensive enough to make it a truely cheaper alternative to buying new and not just away to save a few bucks.

 

[steelespike]

Re: Yamaha 15HP (15D) Prop question

I'm not crazy about prop repair either; but I think what we may have is the motor
was able to rev easier with the damaged prop.
I figure at 21 km your probably not even on plane. I think your weight with one aboard is about 1200 lbs.Over 600 kg.
If you lighten it as much as possible maybe we can get a true planing speed. With a better indication of what prop might work.
Even if we had a tach at this point the motor is turning so slow you would probably be at 3000 or 3500 rpm and you couldn't make up 2000 to 2500 rpm with a prop change. But if we can get a accurate fully planed speed we may be able to guess.(not a good idea) A tach would help a lot. I do think if the motor is sound and we find the right prop speed could be around 20 mph.
I'm thinking that would be a 8 or 9" pitch.
You may also want to be sure the boat hasn't absorbed water over the years.

 

[steelespike]

Re: Yamaha 15HP (15D) Prop question

delete

 

[DennisZA]

Re: Yamaha 15HP (15D) Prop question

Thanks guys

We did the repair as it cost R300 ($28 odd). A new prop is R1700. After the repair the prop is bigger than before by an average of 1.5cm I would say.

I am going to see if I can borrow a lower pitch prop and test. Reading similar threads I reckon I will check the compression too.

I noticed a hairline crack in the fuel filter. Whether this would decrease performance I am not sure?

 

[dazk14]

Re: Yamaha 15HP (15D) Prop question

If it not receiving adequate fuel, that is a problem, although it will leak when the bulb is squeezed.

With the engine idling, do a cylinder drop test... remove one plug wire and check by ear, rpm drop. Do same for other.

You'll need insulated pliers/rag etc.. often time it's easier to already loosen the wire, so it can be moved easily.

If it idles at a low rpm and each cylinder drops similarly, compression's likely fine.

For a Po folk compression test...

Both plugs out, put finger over one plug hole to seal . Pull rope (yes awkward). It should blow finger off easily (even when resisting). Same for other.

If they added a lot of diameter, that will greatly drop rpm.

Have you physically measured the overall diameter?

Please post a picture of the boat and motor. We have fitted some of those yamahas.