prop problems

[tonydiver1]

I have a 1972 forcast fiberglass 17ft boat bowrider it has a 1988 mercury 90hp 3 cyl two stoke problem is. when I got the boat the prop was wreck it was a stianless steel 13x19 pitch prop. so a got a new aluminum 13.25x19 wot 4800rpm. I was told the rpm for that motor was 5600.so I change the prop to a 13.25x17 wot was 4900rpm change it again to a 13.5 x15 pitch wot only 5000 rpm.I thought the low pitch the higher rpm two inch pitch should give 400rpm. I went through 3 differnt pitch and not much has change what am I doing wrong. boat is getting on plane better ,but still not right. not sure what boat weights cant find anything about it anywhere. motor is running well good compression on all cyl, had it all check out. any thought what could be going on. could boat be to heavy for its motor .its what came on the boat

 

[Texasmark]

Re: prop problems

You may have nailed it. May have water in the ballast, or if they just had a sealed floor (which was common back then), water under the deck....had a boat like that once.

You could have the boat weighed, but that requires a lot of research to find the net weight and then deal with what else is on/in the boat when you weigh it. Having changed prop pitch as much as you did and not affected rpm's more than you did says something is dreadfully wrong.

Our family was young back then and we were on the water (as a family with friends and their boats of that type construction) at least once per week. Had a great time. 15' tri hulls were popular then with the 3 cyl OMC engine....could do 35 with a reasonable load, family in the boat, one skier on a pair. No idea what the rpm's were.....we were out to have fun, not take statistics......naw, we were uneducated about proper engine rpms and why you need them. I had an 18' tri hull, very heavy due to hull shape with a 125 and with a prop suitable for skiing slalom pairs, I ran 39 at WOT. I remember these numbers as I could just outrun them and I could never get to that 40 mark.

I currently have a 3 cyl 90 hp Merc, wake pictured in the avatar, so I know what they'll do and they'll do a lot more than you are experiencing.

HTH,

Mark

 

[tonydiver1]

Re: prop problems

just had floor re done no water in there and no bad wood . every thing was replaced. cant find forecast boats any where on line to look up the weight. know one seam to know about them . is it a home made job. it doesnt look like one and has a clean title and says 1972 forecast fiberglass hull

 

[Texasmark]

Re: prop problems

Get your camera out and take some pics, like half a dozen. Want to see side view of lower transom/engine prop, bow shot of the hull, interior pic, rear view of lower transom/prop.

Mark

 

[tonydiver1]

Re: prop problems

View attachment 109658 I dont even have any seats installed yet. I was just trying to set the boat up

 

[tonydiver1]

Re: prop problems

I having a hard time get the pic uploaded

 

[tonydiver1]

View attachment 109678View attachment 109679View attachment 109680View attachment 109681View attachment 109682

 

[tonydiver1]

Re: prop problems

View attachment 109683View attachment 109684View attachment 109685View attachment 109686View attachment 109687

 

[tonydiver1]

Re: prop problems

View attachment 109688 finally got all the pic to go through

 

[Texasmark]

Re: prop problems

Not able to open anything. Get an error message from the site administrator.

 

[tonydiver1]

Re: prop problems

thats sucks ,but I think I found the problem, the motor is to holes to low, causing a lot of drag. got to fix it and retry. will let you know if it works