newbie prop question - advice appreciated

[kwik_uk]

Hi guys,

I'm coming close to finishing my project (see sig) and nearly ready to install my outboard. It is a 1998 Mariner 135 V6.

When I bought it, it came with a 13.5" 23p Laser II on it.

My boat is a 16' Dateline Bikini (a UK Sidewinder clone). It has a 6' beam, and weight I don't know :facepalm:

I've been advised not to take this boat past 55mph as it will chine-walk really badly.


My question is what kind of characteristics can I expect from the Laser II I currently have? Is this prop built for top end?

I'd prefer hole-shot over top speed. With that in mind can anyone advise me on a pitch range? 3 or 4 blade?


Sorry for such newbie questions, I'm just really struggling to get my head around it :redface:

 

[Philster]

Re: newbie prop question - advice appreciated

Laser II is built for top end speed. Has vents to help holeshot.

A 4 blade will make hole shot better, and handle better, but gives up some top speed.

We can't recommend a pitch until you run the boat to wide open throttle; report the top speed AND the RPM she is turning.

Go fasts like yours chine walk. Part of the deal. A 4-blade prop and adjusting engine height can help counter it.

 

[kwik_uk]

Re: newbie prop question - advice appreciated

Hi Philster,

Thanks for the reply. I'm not sure I'll actually be able to give it full throttle with the current prop. I think it will probably push the boat faster than it can handle before full RPM is reached. That's why I was trying to get a ballpark figure of pitch.

Essentially my aim is to have full RPM at just the point where it starts to chine-walk, which I think it about 50'ish kts.

I was thinking something along the lines of a 13.75" 19p Trophy Plus?

 

[steelespike]

Re: newbie prop question - advice appreciated

Speaking generically,you need a prop that will put the motor near its wide open throttle rpm but avoid the chine walk
For the motors health it needs to be able to rev freely within its' rpm rating. You don't have to run at wide open
throttle. Rule of thumb a 1" pitch change will net "about" 200 rpm change lower number increases rpm.
So your 19 might slow the boat but will very likely over rev. A 4 blade prop will give excellent low to mid range performance
should steady the boat some and reduce speed.Normally it is suggested to drop one inch of pitch when going to a 4 blade
to maintain rpm.And the 4 blade will reduce top end.
Its very possible you may have to throttle back or not trim up all the way. You need to trim up some to make the boat handle better as well as make some speed. Obviously you could run your present prop and adjust the trim and throttle so the boat behaves.
Again you can run the motor at what ever rpm you want once you have it setup to run near its max rpm at wot.

 

[kwik_uk]

Re: newbie prop question - advice appreciated

Hi guys, resurrecting this thread as I've finally got the boat out in the water

Well the performance is certainly exhilarating to say the least!! I managed to get it off the clock with my current speedo which is max@50mph.

I think it was up to about the 65ish range by how much extra it had after passing the 50 mark. RPM was at 5200ish.

That is far more top end than I'll ever need I tell you. Still didn't manage to get it as quick as it was capable before I needed to slow down for corners in the river.

One concern I've got is idle speed in forwards gear. The boat is doing about 5-6 mph It's also going up to the top of the temperature gauge. Could this be down to the engine having the wrong prop? I'm going to post a question in the outboards section regarding the temperature, but if you guys can help me I'd really appreciate. Props are a complete dark art to me!

Other than my original description of sizes in the first post, I can't really add anything regarding weights. This is Bumblebee if it helps?

 

[steelespike]

Re: newbie prop question - advice appreciated

Sure is a nice looking little potato ship.
According to the rpm and a 2.00:1 ratio 23" prop thats 51 at 5200 using 10-% slip.
at 6% is 53.2 You might want to use a gps for speed. And we may need to verify the tach.
Does it end up with about the rear 1/4 of the boat in the water.
did you play with the trim a little?

 

[kwik_uk]

Re: newbie prop question - advice appreciated

Sure is a nice looking little potato ship.
According to the rpm and a 2.00:1 ratio 23" prop thats 51 at 5200 using 10-% slip.
at 6% is 53.2 You might want to use a gps for speed. And we may need to verify the tach.
Does it end up with about the rear 1/4 of the boat in the water.
did you play with the trim a little?

I'm going to take my Road Angel GPS unit out next time as it is uber-accurate.

Played with the trim, and if I lift it then it does move faster but I feel like the only thing in the water at that point is the gearbox!!

Could the current prop 13.5" 23p Laser II be putting too much load on the engine at low speed?

P.s it was really funny when I first put it in the water. Forgot the motor was trimmed right up from launching, and when I dropped the throttle the boat literally stood bold upright in the air with it's nose pointing to the sky. That didn't half wake me up!! :facepalm:

 

[steelespike]

Re: newbie prop question - advice appreciated

A 23" prop would be a little faster at idle than say a 19"What is the indicated idle rpm in gear? and in neutral.
If the idle seems fast it may be.Your clutch dog will be happier with a proper idle rpm about 700 in gear.
If its running warm could be the impeller or maybe the thermostat.If you don't know when the impeller was done last you should go ahead and do it.

 

[kwik_uk]

Re: newbie prop question - advice appreciated

A 23" prop would be a little faster at idle than say a 19"What is the indicated idle rpm in gear? and in neutral.
If the idle seems fast it may be.Your clutch dog will be happier with a proper idle rpm about 700 in gear.
If its running warm could be the impeller or maybe the thermostat.If you don't know when the impeller was done last you should go ahead and do it.

Hi,

Impeller is brand new. I took the lower unit off to inspect. All new gaskets, plate and impeller housing. Pick-up tube is making good contact etc. It seems it only gets hot when I'm going slow and there isn't much water pressure. Once I open her up the temps drop right down.

Idle speed as in RPM is fine in gear, it's just the boat speed at this RPM seems a little fast. It's about the speed of a slow jog. Ideally I'd like it at walking pace. I imagine this isn't helping the temps. Feels like trying to pull away in a car in 3rd gear.

 

[kwik_uk]

Re: newbie prop question - advice appreciated



Hi,

Impeller is brand new. I took the lower unit off to inspect. All new gaskets, plate and impeller housing. Pick-up tube is making good contact etc. It seems it only gets hot when I'm going slow and there isn't much water pressure. Once I open her up the temps drop right down.

Idle speed as in RPM is fine in gear, it's just the boat speed at this RPM seems a little fast. It's about the speed of a slow jog. Ideally I'd like it at walking pace. I imagine this isn't helping the temps. Feels like trying to pull away in a car in 3rd gear.

Sorry to bump an old thread, but can anyone offer advice regarding engine temps at low speed? As mentioned in the previous posts, it feels like I'm getting too much forward speed from idle at ~700 RPM. I need to stay below 10 kts on the river, and at this speed the RPM is about 1200. This sends the temp gauge straight into the red.

Is this normal behaviour, or is the prop I'm using putting too much load on the engine at low speeds?

 

[Chris1956]

At low speeds, the themostats control the temp, At higher speeds, the poppit valves control the temperature. If you are sure the impeller is good, replace the 'stats. If you inspect the impeller, look carefully at the bottom of the impeller to make sure it is not chewed up. Any "chewing" would cause low speed overheat.

 

[sullmate]

I have the same motor 1997 vintage on a 18 ft. Searay open bow. I turn about 48 mph at 5000 rpm, using a stainless 13 7/8 x 21 in pitch. Be careful with the auto oiling system on a 16 y.o. motor. I disconnected mine and run 50 to 1 oil gas mix. Peace of mind is a great thing.