Question for walleyehed

[musky49]

I would appreciate your expert advice on this one. I have had porposing issues from the start with this boat, 2003 Crestliner Fishhawk 1750sc, with 75hp 4str yammie. Up front 24vdc troller,2 group 27 batteries & minnkota 210 charger, all in the front storage area, 20# anchor up there in the second compartment. Group 24 starter in the rear port compartment. Prop; turning point 13.25x 15 ss. Results; wot...5550rpm@32mph... with 2 aboard and gear, 10 to 15 gallons in the tank. Front livewell full or empty,,,no difference. I would like more speed, i used to be able to get 34mph@ 5400rpm with the stock prop (13x19k), it should be noted that this was with a 12vdc troller and a group 30 up front,(no charger). I changed props because of excessive cavitation burns on the stock prop, I also tried a 13.25x 17 michigan aluminum (back up prop) with simular results to the turning point)recommended by dealer). I've searched the net and I found a post on here about engine height, I've raised the engine as high as I can without remounting (it gained 2 mph) but didn't help the porposing. I added smart tabs which worked but had to be constantly readjusted for load, so I adapted the Lenco units to work......Recommendations?

 

[steelespike]

Re: Question for walleyehed

Just some thoughts: While all that weight up front helps ease the porpoising it probably makes it nearly impossible to get the bow up a little and keep it there. Trim tabs help control the boat but will eat up speed.
I wonder if the boat has a little bit of rocker in the bottom?You might be able to check it with a straight edge.Have you tried the boat with no weight in the bow?You might take everything out and see how it acts.You could try a prop with less bow lift.4 blade props usually have stern lift
unfortunately they will eat up some speed though in your case might allow more up trim with less porpoising.Usually suggested to go down 1" in pitch when going to a 4 blade to maintain rpm.
Cavitation burns can be an indication a prop is working really well.As might be indicated by
your 34 mph.My info says the 75 is rated for 6,000 rpm as far as speed goes proping closer to that figure may help.Have you checked on the net to see if the fishawk has porpoising issues?
I did a little research and The forum I read no one was running less than 90 hp. and one or 2 said they had a little midrange porpoising issues that went away at speed.Your stuck in that midrange speed.By the way the 90s I think ran 39+ barely touching 40 occasionally so I think your 34 was about right considering the boat is unsetteled.

 

[musky49]

Re: Question for walleyehed

Thanks for your thoughts, all are appreciated, Where I live I am/was limited to a 17' boat with a 75hp outboard. Of course they changed the rules this year to 18' and 90 hp. My 75 is the same motor as the 90 with different carbs and ignition module. I added the module with a 50 rpm gain from it, I'm hesitant to do the carbs without some more info ( are they larger cfm?) I realize part of my problem is that the hull is underpowered ( the boats rated for 125)

 

[steelespike]

Re: Question for walleyehed

One way to to develope the a bigger motor from a smaller brother is to compare the parts sheets.They may be able to help on the Yamaha forum.Be aware sometimes there are exhaust changes as well.You may want to compare the total cost and possible failure against an actual 90.
They do appear to use the same gear ratio.