Prop issues

Ok so my first boat is causing me grief. Ok so the background is 15.5' procraft bass boat 1500lb fully loaded including people I have a 05 mercury bigfoot 25hp 4 stroke when I bought the boat it had a mercury prop 12.5 x 8p the tach would continue toabout 7600rpms at wot.

So I bought a 11 x 15p QL prop and now I do 3600 at wot. I am confused at why such a big change from everything I read I should have been right around 5800. Is this the difference in prop design from mecury to QL

I am going to exchange the prop my two choices are a 11.4 x 12 or a 11.6 x 11. Any suggestions? Also the hub has the rubber things that you put in springs does anyone know what these are for they just don't seem like a good design. Any feedback would be great. (I do realize the boat is under powered it was not a motor/boat combination I picked but I really like the boat and the motor is in like new cond. The lake it was on previously was a 25hp and under.)

Re: Prop issues

Was that 7,600 with a full load?Will the boat plane now?
The rpm specs depend on the boat planing if it can't plane it correctly
rpm wont be right.
Even allowing for the bigfoot a 15 seems high to me.
Have you tried the 15 with a light load?
Was the 8" in excellent shape?
What is the rated capacity of the boat and what is your load?
When you change brands it can throw out the usual
specs.4 strokes need very carefull setup Anti vent plate even with the bottom is usually the starting point.Raise the motor a little at a time to a point just before venting becomes a problem.Small motors usually adding a
1/4" stick at a time to the top of the transom.Be sure it pumps water and has a good grip.With your load this won't give you much but could be the difference between planing and not.

Re: Prop issues

the 1500lbs is a 900lb boat 200lb motor and my wife and I and trolling motor, and some fishing stuff makes up the rest. The 8p prop looked older def well used but there were no bends or nicks. Do 8p not get as much grip in the water b/c of their lack of pitch. I wanted to compare apples to apples so the 2 props had exactly the same load w/ the 8p the boat would plane at about 12mph and although the motor didn't sound like it reved that high at wot I noticed the needle was above 7000 and knew thats not right. With the 15p the boat wouldn't plane just bow in the air it almost seemed like it had too much prop and the motor just couldn't spin it. I need to look at the cap plate it is rubbed so you can barely read stuff...when the rain lightens up I will check that but I was only going to have 2 people in the boat.

I do notice that before plane the motor is pretty deep in the water is that ok? I guess its just normal if the bow is up the stern is down but I almost wanted to trim the motor to get it up closer to the surface but I figured that would be a defeating solution

I don't need tons of speed or a great hole shot but I do want something that peforms. If possible I would like to get to 25mph but I would say all around peformance is the key being able to handle at no wake speeds and being able to just cruise.

Re: Prop issues

OK if it can't plane with the 15 then your rpm would be lower than the 200 to an inch rule of thumb.You should test with the motor trimmed in(prop closest to the boat).For the test run do only one aboard move weight forward if necessary .If you can get it to plane you will have a better idea what prop you need.
If it planes trimmed in all the way it may plane too flat; trim out a notch and see if it will still plane.
Where is the antivent plate in relation to the bottom?
My guess is your motor would only do about 30 with a light fast boat
yours will obviously be slower.I think I would be pleased to see anything over 20+ mph.Weight and weight placement will allways be critical.
I'm pretty sure you will need to lower the pitch but planing with the 15 properly trimmed will tell us better what you need.Let us know what you find.

Re: Prop issues

ok so I went with the 11.6 x 11p prop its a QL at at WOT is does 6000rpm. I am doing 23.4mph so I can be happy with that for sure. I am a little concerned about how low the stern and motor go in the water just before the boat planes or as I am slowing down/ coming off from planing water does not come into the boat at all but it just seems like the motor is low. Can anyone comment on that is it ok.

I also checked the cav plate it is even with the bottom of the boat when the motor is all the way down.

Re: Prop issues

Greenjello said:

ok so I went with the 11.6 x 11p prop its a QL at at WOT is does 6000rpm. I am doing 23.4mph so I can be happy with that for sure. I am a little concerned about how low the stern and motor go in the water just before the boat planes or as I am slowing down/ coming off from planing water does not come into the boat at all but it just seems like the motor is low. Can anyone comment on that is it ok.

I also checked the cav plate it is even with the bottom of the boat when the motor is all the way down.

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23 mph is excellent for that combo.antivent plate even with the bottom is a starting point.if you have no ventilation problems you can raise the motor.
might not give you too much but under ideal conditions might get a mile per hour with a very slight improvement in mileage.
Boats do tend to squat in that phaze just as it starts climbing out of the hole or just as it settles off plane.If you trim in it will come out quicker and squat a little less but may be too flat at speed costing some speed but give it a try.
If the boat seems really low in the stern may be too much weight there.
Probably you have the gastank,battery or batteries, there maybe the anchor.
moving the tank forward would be the easiest way to lighten the stern.
Takes 50lbs out of the stern and adds it to the bow or maybe midway.
Wouldn't hurt to move the battery at least to midway.You could have one of you move forward for starts and stops.

Re: Prop issues

I don't really have much I can move forward. I do have 2 deep cycle batteries one I have found out is bad and will be removing anyway but I can also look into relocating the other battery to the front of the boat in one of the storage areas the only thing I would be worried about is does a battery area need to be vented it seems like the rear area is vented and then I would also need to look into some rewiring I don't know how difficut it would be but I can look into it. Having 2 big batteries removed or relocated would be a wieght adjustment for sure. The gas take is built in and not moveable. I really appreciate all the feedback

Re: Prop issues

If its a fairly large compartment it would probably be ok. Motor charge rate is pretty low so won't be too much venting.If you put a charger on it leave it open.