89 Larson all american dc170 3.7

[dbergeron]

hey everyone, this is my first post on this forum! I picked up a new boat, with the dreaded 3.7l but it was a killer deal i just couldn't pass up. when i got it, it had no prop. i ended up with a quicksilver 14.5x19 3 blade, i figured that might be best for all around performance (i like to go fast but also plan to do a fair amount of towing). So i put it in the water and it planes out fast i mean under 3 seconds with 4, 200lb men onboard, but it revs right up to about 4500 very quickly will a lot of throttle left and im only moving about 30-35 (gps) am i hugely under propped? should i be looking at more of a 21-23 pitch? im not so familiar with boats of this size, and I/Os, but i feel as though this boat should be a fair amount faster than this? any insight would help! bonus points for someone who has a similar set up!
Thanks everyone!
Dan

 

[wahlejim]

Start here
http://legacy.qsprops.com/prop-selector/#

Get the recomendations and get the prop from iboats

 

[dbergeron]

I've tried that but the oldest it goes is 2000 will that matter?

 

[steelespike]

There appears to be about a gazillion props for the 3.7 I do think you may have a combination of under propped and an ambitious tach.
Do you know your gear ratio further research indicates a 1.84 ratio.And wot rpm of 4200-4600. A lightly loaded test run. An actual wot run long enough for a solid gps speed.
or at least a solid run at indicated wot rpm with a solid gps speed.

 

[dbergeron]

i went with only about 250lbs on board for 2-3 minutes at "4400" rpm and boat said 32 mph gps said 34 mph, ill try again on sunday and see if i can get some different readings. I am a carpenter and do minor electrical work (light fixtures switches sockets etc) but ive never done much with DC is there an easy way i can test my tach? it does float around a little so that might be on its way out. though when its cruising it generally appears to function as intended. i did find someone with a like new used 23 pitch for only $60 so im going to pick it up worst case scenario i can use it for my spare. will let you know how things go sunday

 

[steelespike]

Prop calculator says 20% slip at 4400 and 34 mph. Usually slip is about 12% 15% is considered too much on a typical runabout.
High slip often is the result of a tach that reads too high.There may be a selector switch on the back; note the position and operate it a few times to clean the contacts.You could test against an induction tach or borrow a shop tach.We have to keep in mind your 4400 is at partial throttle
Your amazing hole shot sure indicates not enough pitch.Sure would help to see a brief true wot run just long enough for a solid gps speed.
Consider it like running your car up to max rpm in 3rd gear just long enough for a good reading.With your instant hole shot it wouldn't take long for a test.
Does the boat respond to up trim?

 

[dbergeron]

so i put a 23 pitch on last week and brought it out. WOT at 4800rpm showed about 41 gps is this more where it should be? i would've thought id get a little more top end out of it? i wouldve tried a few more passes but i got a really bad vibration so i put it back on the trailer and pulled the drive, gimbal felt good and the ujoints seemed fine. when i put it all back together i fired it up again and in neutral a could hear a clank every few seconds.. it looks like the prop is the bottom of the list now..

 

[steelespike]

Perhaps the coupler is failing?
I think your tach is not accurate, reading high. If we calculate for rpm using 41 mph,1.84 ratio,23" prop and 12% slip we gat about 3900n rpm.
It appears a 21" prop may be in order after you solve the tach issue.