Low RPM at WOT

[clansman]

I've just launched my new boat, she is 21ft 6in excl outboard with an 8ft beam. The engine is an 1994 Evinrude E50RNLIC with a 11-3/4" x 17P prop, don't know what the 'P' stands for.

Took her for a spin in the bay, slowly to begin with and then opened her up after an hour getting to "know her". At WOT the maximum rpm was 4200 at about 10 knots (GPS). I've searched for information on the design rpm and it looks like it should be 4500 to 5500. Any ideas on why she is only delivering 4200 rpm?

I fitted a new tachometer and set it to 6p, from what I could gather the stator is a 12 pole. But I'm not a 100% certain how many poles the stator has. If it were less and I set it at 5 would that increase the rpm?

Thanks for any advice.

 

[hondamech]

You need to be accurate with the engine RPM. 4200 is too low.

The P stands for pitch, your pitch is 17. Looking at your setup it sounds as if the pitch is too high for the engine boat package. Is this a boat without much in it? 21ft would normally be out of range for 50 HP.

 

[eclark53520]

My opinion is you're over prop'd. My 15.5' ranger with a 70hp runs a 17pitch at correct RPM's. I can't iimagine a 50hp on a 21 footer is going to run a 17pitch very well.

 

[clansman]

You need to be accurate with the engine RPM. 4200 is too low.

The P stands for pitch, your pitch is 17. Looking at your setup it sounds as if the pitch is too high for the engine boat package. Is this a boat without much in it? 21ft would normally be out of range for 50 HP.

Thanks for the replies, both. Sorry for the delay in replying.

Unfortunately, it's not just a bare hull. There is a wheelhouse that we built made out of 9mm ply plus glass, 4 off 23litre petrol tanks boxed amidships, two batteries positioned either side at the stern and also an emergency/auxiliary engine a 3.5hp 4 stroke mariner.

I do have an alternative prop which I could try, a 12-1/4" x 15". I also have a spare older 50hp Evinrude which I could add, but that's major surgery and would have to wait until she came out of the water.

I've attached some photos to give a better idea of the bulk.

 

[shawnfish]

lower pitch prop should help but that boat looks a bit much for 50hp in my opinion....any idea of total weight?

 

[clansman]

Sorry haven't a clue as to weight and no weighbridge at this remote location.

Would a couple of tonne be a good guesstimate?

 

[shawnfish]

Sorry haven't a clue as to weight and no weighbridge at this remote location.

Would a couple of tonne be a good guesstimate?

if your anywhere near a couple tons you are way to heavy for a 50hp outboard! even one ton is too heavy, you will wear out that motor and make it work so hard that it is not gonna last long....

 

[cyclops2]

That APPEARS to be a simple work boat hull. That is why there is so much stuff in it.

I do not think there is enough horsepower in the world to raise the speed greatly.

Accept the none planing speed it will run at. Or replace the boat.

Yes. A bigger engine could go faster. But not very much faster.

Are you using Ethanol gasoline ? That causes slower running engines. Try none Ethanol if you can get it. WOT in work boat hulls is only used in emergencies.

 

[guyjg]

As stated you are under powered. That size boat should be running between 100-150 HP. Did you try trimming the engine? You can pick up 200 RPM by doing that. You probably added several hundred pounds of dead weight w/the wheelhouse. Dropping the pitch to a 13P would raise the RPM to about 5000. Get a 4 blade prop for it to increase the lower end power. It will also help at cruising speed pushing the top heavy boat. It will reduce WOT speed but you don't appear to be worried about that. By the way, your photos are very beautiful and peaceful. Where are they?

 

[shawnfish]

either way that's not enough power for that boat even if your just running at half throttle, he is making that motor work waaaay too hard just to move around slowly. that motor will be wore out in no time if it gets used regularly...IMO.

 

[clansman]

That APPEARS to be a simple work boat hull. That is why there is so much stuff in it.

I do not think there is enough horsepower in the world to raise the speed greatly.

Accept the none planing speed it will run at. Or replace the boat.

Yes. A bigger engine could go faster. But not very much faster.

Are you using Ethanol gasoline ? That causes slower running engines. Try none Ethanol if you can get it. WOT in work boat hulls is only used in emergencies.

As you so rightly said it’s a work boat. It was an ex-river clyde fishfarm boat which was powered with a 50 horse Honda. The Honda was stolen and they put on the Evinrude. Then I bought it, stripped it and fitted the “stuff” that was most appropriate for me. The boat will be used for fishing trips and laying creels/lobster pots.
Speed is not important, although as you state useful in an emergency. I was just concerned that the engine didn’t achieve design revs and hence be less efficient.
No ethanol, just pure 50:1 petrol, this is Scotland.

As stated you are under powered. That size boat should be running between 100-150 HP. Did you try trimming the engine? You can pick up 200 RPM by doing that. You probably added several hundred pounds of dead weight w/the wheelhouse. Dropping the pitch to a 13P would raise the RPM to about 5000. Get a 4 blade prop for it to increase the lower end power. It will also help at cruising speed pushing the top heavy boat. It will reduce WOT speed but you don't appear to be worried about that. By the way, your photos are very beautiful and peaceful. Where are they?

I didn’t trim the engine; do you do it before you move off or while underway? Excuse my ignorance.
If I go for a 4 blade prop what pitch do I get, the 17, 15 or 13? As you said speed is not an important issue, efficiency is. I’ve added another photo of the stripped boat to show the keel/bottom line and just for you a photo from my home on the Isle of Barra in the Outer Hebrides, Scotland.

 

[guyjg]

Trim the engine w/the power trim under way...typically after reaching planing speed. The RPM will go up as you raise the engine. Do it a little flick at a time (1 second intervals) untill the RPM start to climb quickley then back it down a touch.

Your country/home are beautifully picturesque. Some day I'll make it to the islands and locate my ancestrial home; Welsh, Irish and Scott. Already have gone back to the 1500s and know where to go look.

 

[cyclops2]

I do not thing planing is in the future. By the time you catch a good load I think planing will not occur. That rounded hull is sturdy but can eat up horse power quickly.
Distribute the work load equipment in a efficient pattern. Then see how much bow up or down weight changes are needed. If you are a cautious fisherman With spots close to shore / port you MIGHT get home in some wind the wrong way.

If going to a distance & willing to stay out for a good haul in sudden high winds & waves ?

Steal the Honda back !!

 

[clansman]

Reading closer you are definately heavy in the bow If you can move the fuel tanks more astearn it would improve handling and overall performance, the wheel house should have been more midship also. Right now you probably just plowing through the water and not reaching a plane. Constantly stuck in the hole?

The fuel tanks are actually positioned centre and aft of the wheelhouse. There are four inline, the bottom two lie aft of the amidships line the other two are the same distance forward of the mid-line. The box container acts as seating while fishing. I positioned it equidistant from the wheelhouse and transom, to allow personal movement around the deck.
As far as the wheelhouse is concerned, when I designed and built it I was advised to put it as far forward as possible. I thought it should go amidships based on beam and weight distribution, but being a novice, I was shot down in flames by the “old timers”. So I decided on a compromise and went in between. It’s much further back than my “advisors” suggested.

 

[clansman]

First of all let me say a big thank you to all who have contributed to my post, it’s much appreciated.
I’ve taken everything that has been said onboard and will change the prop a.s.a.p. I need to beach her for that.

I went for trip today still with the 17P to try the trim method. Onlookers (old fishermen) told me afterwards that at about 8 knots (~ 3200 rpm) she was skimming almost level with the bow marginally raised and slicing through the water. They said she was sitting nicely in the water and looked great.
I gently eased her up to WOT, for about a couple of minutes or so and got 11.4 knots at 4200 revs, I found that at this speed she was pretty sensitive to steer and I thought the nose had risen quite a bit. I dropped the revs to about 3200 and tried the trim method; the gauge flickered ever so slightly up then settled back to 3200 rpm. I didn’t persevere, maybe I should have.

O' and by the way if the person who stole the Honda is reading this.......can I please have it back!

 

[clansman]

If you get the WOT rpms correct that motor will have a long happy life with proper preventive maintenance. A rough guess is 200 rpm for every inch of pitch. The 15 P should raise the WOT around 400 rpms. You really cant tell what any prop will do until you take it for sea trials. Adjust how you load the boat for the best performance you want the bow of the boat to come up a little at first when powering to WOT and then plane out nice and level. Less pitch will also give you more power on the lower end you dont want to lug a outboard 2 stroke.

Just for the record.

I managed to get her beached and changed over the prop to the 12-1/4" x 15". The original prop that was on was a heavy gauge Michigan Vortex Aluminium one.

Took her out for some trials and I couldn't believe the difference. The max' rpm jumped from 4200 to 5100, that's a difference of 900 rpm! And as you said she performs a lot better and is easier to handle. The max' speed at WOT dropped by 1 knot, from 11.4 to 10.4, so not much difference.

Overall I'm highly chuffed, many thanks.