update on speed problem with merc 115

[boaterntexas]

okay took her out, she still boggs down a little when trying to get out of the hole, but once she gets going shes running fine. my 1.5" fiberglass bass boat with a 1976 115 hp merc will get up to about 37 mph, at 5800 or 6000 rpms I knwo the boat shoudl eb going atleat 50 mph, we are not taking on water, have any extra drag or wieght on the boat however I don't feel liek the boat is really planeing out, it still seems that too much of the boat is in the water, but if I look back at the motor, the hydrofoil stabalizer is rigth out of the water with the exception of a the streams commign over the top of the "wing".. this is at full trim up, or as full up as the trim motor will pull it up. my question is, what could be thr problem, the prop? i have in spected the prop, and it seem that none of the blades are curved at teh same angel. the boat is wieghed aroudn 1000 lbs I believe, add me at 150 lbs and my bro at 250 lbs. so you looking at around 1400 - 1500 lbs . where should I start? through my bro in the lake? buy new prop, or check the motor out?<br /><br /> Thanks,<br /> Thomas

 

[DJ]

Re: update on speed problem with merc 115

Thomas,<br /><br />You have to start with a known good prop.<br /><br />Also, that fin may be slowing you down. Remove it for a trial.<br /><br />Your engine may be sitting too low on the transom. You may be able to raise it once you know you have a good prop.

 

[steelespike]

Re: update on speed problem with merc 115

What is the max reccomended wot rpm?<br /> You shouldnt exceed it.<br />Im guessing most of your problem is with your <br />prop.thats a lot of rpm for the speed.<br /> 1.5 inch boat seems awfully small, did you mean 15ft?<br /> get the prop checked and tuned if necessary.<br />Check the bottom of the boat for any bulges,<br />hooks or irregularities.<br /> Move some weight to the back of the boat,it may slow your hole shot but should help the wetted<br />surface at speed.Be sure the motor is reaching max. trim.

 

[Dhadley]

Re: update on speed problem with merc 115

I agree with djohns19 - the fin is probably slowing you down because the motor is too low.<br /><br />You didnt mention the prop size but if we assume the best case scenario of 6000 rpm (we need an exact reading to be accurate) and a 19" prop, theoretical speed is 54 mph. Add typical slip of, say 15% and you should be at about 46 mph. If you have a 21" pitch prop theoretical speed is 59 mph. 15% brings you to 49.5 mph. <br /><br />BUT if you do indeed have a 1.5" boat I would say you most certianly ARE taking on water with that motor bolted to it! We know youre kidding about that - right?

 

[boaterntexas]

Re: update on speed problem with merc 115

1.5" not much room for me, yeah that is a type o its actually 17.5' thanks for your help I wlll get more info on the prop, <br /><br />thanks thomas

 

[mellowyellow]

Re: update on speed problem with merc 115

is the cavitation plate even with the bottom of the boat???

 

[boaterntexas]

Re: update on speed problem with merc 115

the cavitation plate, you mean thoose filling in my teeth. I have no idea what the cavitation plate is . so I have no Idea where it is at. what is a cavitation plate? I want to mesure my rig so I can give ya'll better info on the setup, what should I take messurements of?

 

[rickdb1boat]

Re: update on speed problem with merc 115

 

[boaterntexas]

Re: update on speed problem with merc 115

thanks alot rick very helpfull diagram.. sat. plan to take mesurements, and take some picture.<br /><br /> Thomas

 

[boaterntexas]

Re: update on speed problem with merc 115

here is what I have come up with<br /> from the transom to the very bottom of the boat it is 22.5". from the transom to the cavitation plate it is 20" and the best I can get from the prop looks like 11.5" dia. no telling what teh pitch is however none of the blades seem to be curved the same here are some pics. okay I can't seem to get a pic to load, so give me some time then I'll add them<br /> thanks for your help guys<br /><br /> here is teh best I can do, for some reason it wont let me link directly to the pics so thees are small and you can't tell much but take a look and see waht you think<br /><br /> boat pics <br /> <br /> to get the full size pic, cut and past this addresses http://www.geocities.com/tlcnnetex/cavplatjpg.jpg <br /><br /> http://www.geocities.com/tlcnnetex/draw.jpg <br /> <br /><br />thanks for your time