15hp mercury 4 stroke prop question

[hwindle]

I just bought a 2005 Sundance skiff, with a 2005 Mercury 15hp 4 stroke engine. This boat weighs 700 lbs (hull only), and is rated for 50 hp max, with 40 recommended. Needless to say, it is underpowered. The prop, which is probably stock, is aluminum and says A12 9P, which I think means the pitch is a 9. I bought one of those cheap little universal RPM gauges that have their own battery, and you wrap the wire around the spark plug wire. We took it out today with a GPS to measure speed, and at WOT it did 10 MPH at 4400 RPM's. It doesn't plane, so backing off the throttle only slows you a little.
The prop is chewed up a little, so I want a new one anyway, and I think it should be less pitch. It vibrates at idle in gear a little, which goes away at around 1/4 throttle. The motor vibrates in neutral too, which I don't think it should, but I have no experience with these motors.

Can someone tell me:
1. What do those numbers on the prop mean?
2. Should I drop the pitch to get higher RPM's? I think they are supposed to be around 5,500.
3. Since I am plowing through the water anyway, would higher RPM's give any better performance.
4. Should these motors vibrate at idle, in neutral? I pulled both plug wires, one at a time, and it runs the same on either cylinder.
5. Would a 4 blade prop do any better?

At idle in gear, it trolls just fine for over an hour at a time, without packing up at all. Once started the motor will instantly restart, and doesn't miss or any other bad habits. We trolled today, with a couple of higher speed runs, for 3 hours, and used around 1 gallon of gas.

Thanks.

Harry

 

[steelespike]

Re: 15hp mercury 4 stroke prop question

Is this a tiller setup?If you look under the prop nut the numbers may be easier to understand.
It sure would help if you could get it to plane. Perhaps move the tank and battery forward for the test.
Maybe a light partner to hold the bow down, Try trimming in a notch and move some weight back when it planes.
At 4400 not planed makes your rpm artificially low. Little 4 strokes kind of need to be propped near the high end of the rpm range
6.000 in your case. You can't really make up 1600 rpm practically with a prop when your already at 9". It would likely knock the heck out of your top end speed.Can't help with prop numbers except for 9" pitch.You no doubt will need a lower pitch. If we can get it to plane it will be easier to tell what you need. Just a few pounds in the right spot can be the difference to plane. On some boats just leaning as far forward as possible can be the difference. A 3 gallon tank instead of a 6 can help,Eliminate as much weight as possible.
With a 4 stroke you have half as many power strokes of twice the power per stroke.So a little vibration may be normal.
Also some 2 cylinders the pistons go up and down at the same time like a Harley Davidson ,did you ever see how they vibrate.
Yes a 4 blade could help low to mid range performance,usually at a slight sacrifice of top end.

 

[anzomcik]

Re: 15hp mercury 4 stroke prop question

I played around with a 9.9 on my 14 foot boat (thats is what the boat came with) I bought several different props, different trim settings, got a tiny tach, lifted the motor higher on the transom, shifted weight around... No matter what i did I could not plane the boat out.

I would put money on you will not get any more preformance, and notice very little change in speed and RPM by switching props. That motor is only going to move the boat at displacment speed. You will always be at 10mph.

For slow trolling like you mentioned, it will be perfect for that. However if you think a prop will get you on plane, you will be disapointed. But if your prop is in very bad shape a replacement would be good to save wear on your motor.

If you want to go faster, do what I did, buy a bigger motor

 

[hwindle]

Re: 15hp mercury 4 stroke prop question

Thanks for both your imputs. This boat has a built in gas tank in the center consol, and having my wife move from the stern to the bow makes no difference in speed or the tilt of the boat. I agree that this is too small a motor to get this up on plane. I think I will get an 8 pitch, just because it doesn't rev up as much as it should, and I need to get something anyway.

 

[steelespike]

Re: 15hp mercury 4 stroke prop question

I feel it should plane the boat unless you have a lot of extraneous weight.
I checked some performance reports a 20 hp planed a 525 lb Mirroctaft with a 10" prop. at 21 mph a @ 5500 rpm.
And a 15 hp did a 424lb Boston whaler with a 9" prop at 20 mph and 5500 rpm.
The ease of these tests seems to indicate your set up though heavier should manage with a struggle.Perhaps the 8" will get it done. It appears there is no need to fill your on board tank That could save about 6lbs per gallon.
If you can get it to plane you could save gas because you can get from here to there faster perhaps at reduced throttle.

 

[hwindle]

Re: 15hp mercury 4 stroke prop question

Well I got my new 8 pitch prop, and went out yesterday. The motor went from doing 4,400 flat out to 4,900 now. But the speed dropped from 10 mph to 7 mph. 3 mph isn't a big drop if you doing 30. but it's 30% slower when you only doing 10. I had maybe 100 lbs more in the boat than before, and neither the increased engine speed or the decreased boat speed was noticable without the GPS and tach telling me actual speeds.

Do you think I should have the 9 pitch rebuilt and put it back on, get a 7 pitch, or leave the 8 on the boat?

 

[steelespike]

Re: 15hp mercury 4 stroke prop question

Remember I said you really can't make up 1600 rpm difference with a prop and it would knock the heck out of the top end.
I think you need to confirm the motor is sound,that the throttle opens all the way,good compression,Antivent plate is about even with the bottom of the transom.Did we check that the bottom is clean and smooth? Maybe a plug wire is off.Maybe thats why the idle is rough.

 

[hwindle]

Re: 15hp mercury 4 stroke prop question

The bottom is clean and smooth, and it idles the same on either cylinder, so they are at least balanced. The plate is about 1" below the bottom of the hull. I did drill a hole into the foamed bottom, and drained around 6 gallons of water out, which should lighten things up around 50 lbs. I haven't checked the throttle, but I will when I change oil and check the valves. It starts immediately, so I don't think that things could be off very much. It is just under powered. Good think we aren't in a rush to get any where.

 

[anzomcik]

Re: 15hp mercury 4 stroke prop question

If your 9 pitch is banged up enough you think it could cause damage to your motor, keep it off. The new 8 pitch should troll you a tad slower. A 7 pitch will not get you anymore speed, you are going to be at the upper end of your displacment speed with that motor.

With my old set up that was similair to your, I had 9 ptich, 7 pitch, those bolt on hydrofoils, changed load distribution, motor height, tilt angle... anything you could think of to get my boat to go faster with my 9.9 merc. It would not go above 8.5mph with gear and another person it was upper 7mph.

Personally I feel in this situation you have what your going to have, no sense in changeing things. if the motor cant plane you out with your first prop that it is such a long shot that small tricks will get you to plane out, I learned that first hand, I had a blast doing it but i did not get the results i hoped for.

So enjoy the boat you have, Some of my fondest memories were when I didnt have the fasted boat out there, it was when i was slow and enjoying the ride i had. it was simplier back then.

But if you do have the want for going faster, a new motor is what you need. At least a 40hp like your boat says.

 

[steelespike]

Re: 15hp mercury 4 stroke prop question

The bottom is clean and smooth, and it idles the same on either cylinder, so they are at least balanced. The plate is about 1" below the bottom of the hull. I did drill a hole into the foamed bottom, and drained around 6 gallons of water out, which should lighten things up around 50 lbs. I haven't checked the throttle, but I will when I change oil and check the valves. It starts immediately, so I don't think that things could be off very much. It is just under powered. Good think we aren't in a rush to get any where.

If you drained 6 gallons of water very likely the foam is still saturated with as much as a couple of hundred pounds of water.
Once saturated the foam needs to be replaced so it can do its job and also lighten the boat.

 

[hwindle]

Re: 15hp mercury 4 stroke prop question

Thanks for the reply. I wrote the factory, and was told they put 21 different foam filled compartments in the hull. So draining any other water, which I know there is some in front of the helm, is going to be a problem. They suggested drilling holes in the hull and patching after draining, but I would think most of the compartments are still dry. I drilled a hole to the port and starboard in the bilge, and they were dry. Just the center had the water at the stern, but when drilling a pilot hole for a floor mount in front of the helm, water came straight up. Not like a fountain, but at least a 1/2 cup came out.

Replacing the foam is a no go. Way too much work for a guy like me. I wish there was a way to tell where the water is and isn't, but I can't think of a way without a lot of drilling a patching. I will get it weighed, and see how much heavier it actually is first. There are worse things then going slow, and another motor will cost me more than I have in the entire boat. I would think selling a 15 hp without tiller steering is going to be hard.

 

[steelespike]

Re: 15hp mercury 4 stroke prop question

Weighing is a good idea. At least then you'll know if weight is the culprit. Give the motor a good checkup.
Raise the motor to a point just before venting becomes a problem.At least that 1".Wont add much speed but might be enough to get it over the hump. If you get the height right then a hydro foil might help without adding to much drag once planed.
On a light boat the 15 could make about 27 mph If you could get it to plane you could about double your present speed
saving gas and time.Your motor will be a lot happier within its rated rpm range.Keep us posted.

 

[hwindle]

Re: 15hp mercury 4 stroke prop question

Well I weighed it, and it was around 365 lbs. heavy. I cut a 6" hole in the front where water had come up from drilling the deck, and hit two different chambers. One was wet and one was dry. I have attached a picture of how they make the hull. A great design, unless water gets in any of them. That problem I think needs to be on another thread, in another forum.

My last trip with around 50 lbs less weight in passanger, the motor didn't hit the 4,900 it did before, but only 4,680. That doesn't make sense, and makes me think I need to drop another pitch, from 8 to 7. I have read somewhere that you should at least prop to where you get around 5,500. One less pitch should only increase about 400 RPM. It could go slower at idle and not bother me a bit trolling.

 

[anzomcik]

Re: 15hp mercury 4 stroke prop question

Ok was the boat hull 365lbs over weight? so it is over 1000lbs? Did you have the motor, battery, gear, gas and anything else in the boat when it was weighted.

As for your RPM alot of things effect that, did the motor run the same as it has before (seat of the pants feel, same vibrations and sounds, speed)

One inch of pitch rule of thumb is 200rpm, not 400. Whats the lowest pitch you can get. Lastly what is your lower unit gear ratio?

 

[steelespike]

Re: 15hp mercury 4 stroke prop question

If its 365 heavy and you run with 2 aboard its like having 4 adults aboard. Your rpm is not a true indication of what the motor is capable of because its not planed out. The lower rpm may be an indication of some sort of motor problem.
If you do get it to plane a 7" prop will likely over rev but at 5500 will get about 16 mph that's only 3 mph faster than not planed.
A 9" prop should get about 21.

 

[hwindle]

Re: 15hp mercury 4 stroke prop question

Yes, I figured all gear, motor, gas, etc. when determining the weight. The whole rig weighed 1,800, and the trailer weighs 460 lbs. I have a 7 pitch ordered, and am in the process of cutting holes and pulling out wet foam. I don't see how I am going to even get to 100 lbs removed, but at least the 6 gallons I have gotten so far compensated for the new seat I put in. The deck is about 3/4" thick fiberglass, and it eats jig saw blades. Wiped out 2 of them cutting a 6-1/2" round hole for a screw in hatch cover.

Thank you all for your responses. Now I just have to cut holes and play around with it. With 3 different pitch props, and the GPS and tach, I will get to the optimum sooner or later for my little hole in the water.

 

[steelespike]

Re: 15hp mercury 4 stroke prop question

You may find a lot of the water concentrated toward the stern of the boat where it does the most damage.
Have you weighed the foam you have taken out?
Does the foam provide structual support to the hull? If its all sealed up where did the water come from.
Foam provides flotation if the boat sinks. You may have to replace it both for support and flotation.
I believe there is closed cell foam that can be sprayed in.
If the foam isn't structural Maybe you can just make air chambers.
Maybe you can find demolition blade or perhaps a coarse blade for metal cutting.

 

[hwindle]

Re: 15hp mercury 4 stroke prop question

I have ordered some large hatch covers, and will cut out sections of the floor and put them in. The foam isn't as structual as the fiberglass making the different compartments that connects the hull with the floor, so I will leave most of them in and just hole saw to get access to another one. I pulled foam for 2 hours yesterday, and got about a half of a large garbage bag, that weighed around 5 lbs. That was in front of the helm. I am going to cut 2- 13" x 18" hatch holes near the stern, and that'a where I expect to find the most water, and have a large opening to work with. I will probably put those pool noodle things back in for floatation, since they don't ever absorb water. The foam I took out was water logged, even at the top. Closed cell foam still absorbs water, and it don't want to come out. Luckily I have no wood, so no rot.

Thanks for all your help.

 

[steelespike]

Re: 15hp mercury 4 stroke prop question

Hopefully You'll have a different boat when your done.
The noodles sound like a good idea. Don't know how many you'll need but you can get them here for about a buck apiece.

 

[steelespike]

Re: 15hp mercury 4 stroke prop question

Making any progress?