wot question

[peeterb]

i have read many posts with the search feature with no answers to my question.<br />I have a 1989 fiberglass v hull 16 foot boat that weighs 1600 lb with a 1986 110 hp evinrude.My wot is around 6100 rpm at a speed of 41 mph.I currently have an aluminum prop 13 1/4 by 17.This rpm sounds a little high should i try a different prop?<br />Thanks for any help.

 

[jimmbo]

Re: wot question

your engines range at WOT is 4500 to 5500 so adding some pitch is in your future. Your engine actually makes it peak power near 5400 rpm. try a 19 or even a 21 your speed should go up a couple of mph too.

 

[peeterb]

Re: wot question

before i go adding more pitch should i try adjusting the height of the motor?At this point the antiventilation plate is within a 3/4 of a inch of the bottom of the keel when trimmed at the trim line.Also is there a mathmatical formula that will tell me how many rpm i will loose for each number of pitch i increase.<br />Thanks for the reply.

 

[angus63]

Re: wot question

appx 200 rpm loss for every 1" increase in pitch everything else being constant.

 

[peeterb]

Re: wot question

This question is for Jimmbo.Why do you give such a wide range for wot?Does this mean if going to a 21 pitch and getting 5200 rpm wot would be okay for the engine and not lugging it?<br />Thanks for reply.

 

[peeterb]

Re: wot question

I want to reword a previous post about the antiventilation plate.The antiventilation plate is exactly 1 inch above the keel of the boat when trimmed level.Does anybody know if this is to high or to low ?

 

[ob]

Re: wot question

If the engine performing fine at that height setting and you are not experiencing excessive prop ventilation throttling out of the hole or turning maneuvers ,leave it.Your WOT rpms are just a bit too high.You could experiment with a prop of steeper pitch and water test to see what effect it had on your WOT rpms.There are some 18" pitch props available which may just put your R's and top end in the sweet spot.See if you can locate a shop in your area that will let you water test one.Shops in my area will, and if you don't scuff or otherwise damage the prop they will gladly let you return it.

 

[Dhadley]

Re: wot question

Youre at about 18% slip now. Thats not too bad but it could be a bit better. Is 6100 with a light load or an average load? We target 5800 with an average load for that crossflow.<br /><br />The actual X dimension for a given motor depends on the technology built into the prop and how it reacts to a given hull design. Thats why some run higher than others. <br /><br />Is your prop in good, unworked condition?

 

[peeterb]

Re: wot question

I have two props with the same dimensions and they are both in great shape such as no chips or bends or dints.the two props seem to perform the same when on the motor.The second prop i just puchased from a local prop shop and yes it is a rebuild.When i have tried the props out this is with myself 185lb and two boys 125lb and the other 90lbs and 10 gallons of fuel.I would say this is average load.Should I change pitch or lower motor or try anything else.<br />Thanks for any ideas.

 

[peeterb]

Re: wot question

OB thanks for the reply.<br />Do you think dropping the motor down one hole would help lower the rpm a bit.

 

[ob]

Re: wot question

It would lower the rpm alright(around 150 rpm per hole) but most guys would kill to have your problem and would look at instead of lowering the engine (which will icrease exhaust back pressure and drag),to opt for a steeper pitch prop and gain performance all around.Another option if you choose not to increase in prop pitch is just to not exceeed 5800-5900 rpms at wot.I personally would experiment with some other prop choices.<br /><br />This is all considering that you are getting accurate tach measurements and your current setup is not experiencing excessive prop ventilation.

 

[peeterb]

Re: wot question

Ob when I run the boat in the water what is the maximum rpm a person should run the motor at.If i run at 5800-5900 rpms and not exceed this range will i do damage to the motor.Out of the hole when i start at full throttle is it all wright that the rpms hit 6100 rpms for a couple of seconds until boat on plain or could this do damage.How do I know if i have excessive ventilation.

 

[ob]

Re: wot question

Any time you exceed the rated rpm of the engine you are treading on dangerous ground .You'll know if excessive ventilation is occuring by performing a few tight turning manuevers at around 4500 rpm on plane and observing whether the prop loses bite which results in the engine rpms increasing rather quickly.Some guys refer to this as prop blowout.It can also happen if the engine is simply trimmed up too high during the maneuver or getting up on plane.Each boat and motor setup has it own uniqueness with respect to trim position and engine height placement.

 

[peeterb]

Re: wot question

When you say tight turn how tight.

 

[ob]

Re: wot question

As tight as possible while still maintaining a good level of control.This displaces water around the transom ,and if the engine is mounted too high ,the prop will begin to ventilate.If this ventilation is such that the boat loses measurable speed and the rpms shoot up,you're too high.You'd actually like some "slight" ventilation at this crucial point.The engines trim angle also comes into play,so you'll need to get a feel for what is best.The higher the trim angle,the easier the prop will ventilate.

 

[peeterb]

Re: wot question

I know that if i have the trim set at the level line and turn the boat real hard that it over revs all of a sudden.this happens when i am at about 5500 rpms.weather permitting i will take the boat out tomorrow and give it a wirl at 4500 rpms.I will let you know how it goes.

 

[ob]

Re: wot question

Just be careful with the amount that you attempt to turn the rig.Particularly at high revs.Don't want you to highside it or anything.Just get a feel for where its at now.<br /><br />I still feel like your rig is slightly underpropped by the 6100 rpms you are getting at wot.

 

[jimmbo]

Re: wot question

4500 - 5500 rpm is the range the manufacturer speced. Best top speed will occur if you can get a prop that will let it turn at its peak horsepower, which I think is about 5400. Overall best performance will be obtained if you prop it for 5500 with a light load. In theory you should be able to prop it for 4500 and it shouldn,t melt the pistons or otherwise suffer for it but...

 

[peeterb]

Re: wot question

made it to the lake today and here are the results.13by 19 imch pitch would do at wot 43 miles per hour at 5900 rpms.would cruise nicely at 5400 rpms at 40 mile per hour.Turning real tight the prop would not ventilate but after a real tight turn and then straightening out it would then cavitate a bit only when i would straighten back out.<br />Second prop stainless steel 12 3/4 by 21 inch pitch wot 5300 rpm @45 miles per hour.<br />This prop was great no slip on turns and hole shot was still pretty good.Theese tests were done with the same as previous myself and my two boys.Does this stainless perform enough so that the rpm are not to low?<br />Thanks

 

[peeterb]

Re: wot question

let me reword the question.Is 5300 rs high enough for this motor at wot or will i be lugging it and causing damage down the road?<br />your thoughts apreciated.