Sluggish four stroke - new prop?

[longshanks]

Hi folks

I'm running a 14.5' welded heavy-duty aluminum fishing boat. It's a 5 degree, beamy hull, with a small 'tunnel' in the last 20" or so of the keel, designed to run very shallow. I bought it with a 60/40 mercury 2-stroke jet drive (tiller handle), which I GPS'd at around 42 km/h wide-open throttle. This setup takes around 7 seconds to jump onto plane.

I recently bought a second motor for this boat, a 2006 50 HP Mercury four stroke prop motor. After taking it out for a test drive, I'm not overly impressed with the performance of it, so I thought I'd post here and see if you guys have any ideas on whether I can improve it, or if I'm out to lunch...

As the motor is run by a tiller, I have no idea about RPM's, ie. no tach. It sounds like it is running perfectly, but doesn't sound as if it is revving very high. Engine is mounted with the anti-ventilation plate level with the bottom of the keel. I have started with a pirahna composite prop, 12" x 11C. This setup takes around 10 seconds to plane, and my GPS'd top speed is 36km/h. Considering the 10 additional horsepower over my jet pump (40HP at the pump), I was hoping for something more like 50km/h. Is this unrealistic?

Any suggestions on where to head from here? I have considered installing a tachometer, but if there are obvious better prop choices, maybe I should try them first. Merc recommends a 11" x 12C prop, or thereabouts.

thanks in advance for any suggestions...

cheers

longshanks

 

[Rancherlee]

Re: Sluggish four stroke - new prop?

I suggest getting a tach first, even if you just rig up a temporary tachometer for prop testing. You want to run it at the high end of the operation range (6000rpm in your case) I've noticed 4 strokes are prone to being slugish if you are running close to or below the lower end of its WOT rpm, but if you prop them to run near or at the top of there WOT rpm range then they perform quite well and keep performing well even as your load increases.

 

[longshanks]

Re: Sluggish four stroke - new prop?

Thanks Lee for this response. I am working on finding a wiring harness and tach for my boat.

I found some boat tests with 50HP four strokes, on the www. These tests claim that a properly set up boat should do around 46-50 km/h, and get onto plane in 4-6 seconds.

My boat is topping out around 36km/h and it takes over 10 seconds to get on plane. I would have thought that if I was using an improperly sized/pitched propellor, the boat would either have good low end torque, or high top end speed, not lack of both. My present set-up seems to be lacking both. Is this still a problem that could be caused by poor prop selection?

longshanks

 

[Rancherlee]

Re: Sluggish four stroke - new prop?

you will find that with the proper prop you will have both good hole shot and good topend. I have a 70hp 4 stroke and Just got done finding the perfect prop for my setup. I tried 4 different props before I found a good one, and the best one had the best hole shot and topend speed.

19 pitch omc cupped aluminum (came with the engine)
hole shot, SLOW 10+ seconds
Topend 35mph @ 5200rpm WOT
really slowed up with extra weight, 31mph @ 4700rpm with only 2 extra people

17 pitch OMC no-cup aluminum (had it sitting around)
hole shot Good 6 second
topend 36mph @ 6000rpm
34mph @5800 with 2 extra fishermen + gear +livewell full

18 pitch Turbo 1 vented stainless (used, good deal off ebay)
hole shot Awesome 4-5 seconds
topend 38mph @5800rpm
36mph @ 5600 with 2 extra fishermen + gear + full livewell

Off course your setup is different then mine but this seems to be pretty consistant with oter 4 strokes, if they only turn at the bottom end of the WOT range your performance is going to be pretty mediocre.

 

[longshanks]

Re: Sluggish four stroke - new prop?

I appreciate your response Lee. Both of the props I've tried thus far have been only mediocre in my opinion. I am reassured that I may be able to get some improvements, by getting a tachometer reading and then going about prop selection.

My boat is quite heavy, there is no doubt. When my girlfriend and her mom were in the boat (not saying they are heavy!), I noted that it took considerably longer to get on plane, maybe up to 20 seconds, and top end speed decreased by 4 or so km/h.

Once I have some WoT RPM readings, I may be back to ask some more questions.

regards

longshanks

 

[studlymandingo]

Re: Sluggish four stroke - new prop?

When my girlfriend and her mom were in the boat (not saying they are heavy!), I noted that it took considerably longer to get on plane, maybe up to 20 seconds, and top end speed decreased by 4 or so km/h.

Better watch that one, if she ever sees this post you could have much bigger troubles than slow plane times!!!

Where is the A/V plate in relationship to the bottom of the hull? If it is sitting low, raising the motor a bit could help the overall performance. I never gave this much thought until I did it to my own rig, made a huge difference.​

 

[longshanks]

Re: Sluggish four stroke - new prop?

My hull is unique, as it has a small tunnel, about 10" wide and 18" long. The top of the tunnel is angled upwards from the 'V' of the hull, to the top rear edge of the tunnel, and it runs beautifully with a jet shoe on my other outboard, mounted flush with the trailing edge of the tunnel. The other thing I should mention is that the hull is heavy (never weighed it yet), and flat like shingle (5 degrees or so), so I may not exactly get blistering performance out of a 50 hp 4-stroke on this particular hull. But hopefully better than 22 mph...

I have the (pictured) prop motor mounted so that the A/V plate sits around 1.5" beneath the top of the tunnel. The boat manufacturer recommends that the A/V plate should sit flush, or .5" below the top of the tunnel, but when I initially tried that, my motor seemed to have some issues with water uptake, BUT I should mention that I was having some serious issues with this motor, and it was rebuilt under warranty. I am now working with essentially a brand new motor, and I don't want to fry it! As I have no water pressure gauge, it is difficult for me to tell if the motor is not getting enough water. I have mounted the motor in its current position, hopefully erring on the side of caution. However, there is a fair bit of spray coming off the motor leg, and I am wondering if it is too deeply set, contributing to the slow planing and slow top end I am experiencing.

I really appreciate your feedback, this site is a great resource!

 

[steelespike]

Re: Sluggish four stroke - new prop?

If you can raise the motor 1/4 inch at a time perhaps someone can watch the telltale to be sure it pumps.If it is indeed dragging you could pick up 1 to 3 mph maybe more if it is really down.If you check the water intake position you may be able to tell how high will work.In my opinion the tunnel can cause more turbulance than a plain bottom.You may need to do a jack plate to get the motor back more and make it easier to fine tune the height.
It may be difficult to get the hull up on the trailing edge some props may help with that.

 

[longshanks]

Re: Sluggish four stroke - new prop?

I had the motor to my local shop today, and they hooked it up to a tach, and determined that my max RPM is around 5700, @ WoT, with one person in the boat. With typical loads, of 2 or three, this figure comes down lower, and the boat takes around 20 seconds to get on plane.

This is using a Pirahna composite prop, 12" by 11 pitch. The only other choice for blades on this prop is a 12 1/4" by 9 pitch, which should help me get the RPMs up to max (6000RPM), and will likely help the holeshot. But even with the current 12x11 prop, my top end speed is fairly unimpressive, at around 38km/h with one person.

Looks like I may have to play around with different props, and possibly the height of the outboard, although my mechanic figures it is set in an ideal position presently.

Any suggestions?