Am I overpropped?

[Flick40]

I have a 14 foot fibreglass "sea nymph" powered by a 40hp Jonhson outboard.<br />With one or two people in it the boat will plane happily at 35km/h, but<br />struggles to get up on the plane with three people and can't get up with 4.<br />I was told a cavatation plate would help but after fitting one, it has made<br />no difference. With 4 people the outboard will only get up to 3000rpm - it<br />lifts onto the plane at 3200rpm. If I changed the prop to one that is 2<br />degrees less pitch, that should lift the rpm by 400, presumably then allowing the boat to rise up onto the plane. With one or two<br />people on board when planing, the engine will run at about 6000rpm. My question is really - is it over propped?<br />Would a prop with less pitch fix the problem? most boat shops just want to sell me a bigger engine, but the engine I have is a beauty, and I am unwilling to look at the considerable cost of replacing it with a bigger one!

 

[snapperbait]

Re: Am I overpropped?

Yes a prop with less pitch will definatly help matters...In your case I would go down about 2 inches in pitch.. If you could list the model year of the engine or it's model number, the diamater and pitch of the prop you are currently using and approximate weight of the boat you might get some better responses....

 

[ob]

Re: Am I overpropped?

In a boat of that size and horsepower it might behoove you to have two props .One for heavy load applications and the one you now run for lighter ones.Depending on how often you plan to carry 4 passengers.

 

[ob]

Re: Am I overpropped?

In a boat of that size and horsepower it might behoove you to have two props .One for heavy load applications and the one you now run for lighter ones.Depending on how often you plan to carry 4 passengers.

 

[petryshyn]

Re: Am I overpropped?

Its hard to be propped for all occations....<br />Since you already are getting max rpm when planed, you simply want to improve your hole shot, right?<br /><br />Do a search on "ventilating prop"<br /><br />Then read this Click this <br />Some interesting reading that may get you that sweat spot!

 

[Flick40]

Re: Am I overpropped?

The engine is a 1988 40hp Johnson, <br />serial No Ao 7603316. The prop has stamped on its inside 12 1/4 X 15.<br />I don't know what the boat weighs - It is a Sea Nymph escort, it is open, no cabin, but has a fglas flat floor in it -14 foot long, fairly sturdily made - maybe someone out there has something similar and would know its weight.

 

[suzukidave]

Re: Am I overpropped?

Flick40, first off I wonder if your tach is accurate? You should be much faster than 35 kmh (21 mph) at 6000 rpm with a 15" prop on that boat/motor setup if that's a standard planing hull. It should be closer to 35 mph (55 kmh). <br /><br />As for the refusal to plane with extra weight, a 15" prop is a bit large for a 40hp motor so dropping pitch 2" would help but an 11 or 12 inch is probably optimum for heavy loads -- unfortunately it will also make your motor over-rev with two people in it. Before you do that try trimming the motor in as tight as you can to the transom and also making sure all the weight is shifted forward when you start. If you can get the boat on plane you will have a truer idea of the wot rpm of the 15" fully loaded - 3000 rpm just means you can't get her on step -- it will build up more rpm if you can get her up there.

 

[Flick40]

Re: Am I overpropped?

It is a standard planing hull- it is known here as a gull wing design - don't know about the accuracy of the speedo - could be travelling about 50 k/h.

 

[93evinrude]

Re: Am I overpropped?

I you do get a second prop you will be changing props every time you reduce passengers to stay within rpm range or you can make sure you stay within range while carrying fewer passengers which will result in less top end speed.If either is alright then you could do it but I would look at a couple of other things first.<br />If you do not have power trim,make sure that the engine is at it's lowest tilt position,meaning tucked in as much as possible as it is easily adjusted.If it can be tucked in more it may be all you need along with a possible weight distribution change.<br />Also,where is your prop shaft in relation to the bottom of your hull.

 

[Flick40]

Re: Am I overpropped?

engine has power trim + tilt, it has been spaced with sort of v brackets to get the angles right -done by the outboard dealer-mind you, he also supplied the prop! prop shaft is about 9inches below keel. relised after reading comments that i keep the fuel tanks - 20litre x2 right aft in the boat under the rear molding. moving these forward might alter weight distribition.

 

[93evinrude]

Re: Am I overpropped?

It sounds like your motor may be mounted a little lower than typical and lifting it may also help if you have adjustment left without re-drilling.Where is your cavitation plate relative to the bottom of your boat?<br />Are the v wedges tilting the lower section of the motor away from the boat or angling it towards the boat?<br />Any other comments from posters?

 

[MGuckin]

Re: Am I overpropped?

Try as schematic says and vent it.<br />You won't rev higher but will rev quicker.

 

[Flick40]

Re: Am I overpropped?

v plates angle the lower end of the motor towards the boat, cavitation plate sits right in line with the keel.

 

[suzukidave]

Re: Am I overpropped?

flick40, your cavitation plate is is the right spot. <br /><br />It sounds like you or dealer have done several things to try to improve holeshot while keeping top end performance. smaller boat/motor combos are susceptible to big performance changes from increased weight. I had similar issues on my 16'/55hp though not as bad when I tried to increase pitch. You have good performance with 2 people on the boat, so you don't want to lose that by permanently dropping down in pitch for the occasional heavy load. Here are all the suggestions I can think of short of switching props all the time:<br /><br />-move all weight forward when you have a big load<br />-trim motor all the way in (sounds like you did this)<br />-jack plate to increase angle of motor (sounds like you have it)<br />-add a hydrofoil (soudns like you have tried)<br />-see if a 13 or 14" prop could be an "all rounder" without losing too much top end performance or over-revving<br />-try a 4 or 5 blade prop at 14 or 15" (they give better hole shot than equivalent 3 blade without losing much speed)<br />-ventilate prop as schematic suggests<br /><br />You also might consider manual trim tabs. I love'em personally and think they are unreal for small boats like ours for a bunch of reasons but they cost about C$200 and are still pretty new, so not many people use them. They are much better than a hydrofoil for improving planing without increasing drag. My boat jumps on plane with them, even with a heavy load (I had 9 people in the boat this summer without planing problems after another boat broke down). If you are interested let me know and I'll give you a spiel....