prop size help

[splitshot]

i have a 1986 hydra sports bass boat 270x i believe is the model...it has a 1977 140 johnson...with me, my father in law, fishing gear, 2 batteries for trolling and 1 for cranking the max rpm i can get is around 3000 and it does about 35 mph (gps) this is after tuning it up and playing with the tilt and trim on the water this is the best it will do....the prop is 13-1/4 X17 the motor runs great it just dont have the rpms or mph...looking to change the prop if it will help....any help is greatly appreciated

 

[steelespike]

Re: prop size help

You may want to check your tach seems low for the speed And the speed seems low for the boat/weight,motor combination.I'm assuming a bass boat is a fairly fast design.A quick look around that combo seems to run 40 to 48 mph with a boats in the 1400 lbs dry range and a 17" pitch. Boat motor combo close to 2,000 lbs.
The dry weight of your boat is 850lbs and its 17 ft didn't find a 86 270 but an 85.Does the boat respond to trim.I would check the boat for absorbed water,is the bottom clean and true?is motor timed right,throttle opening all the way,compression?Good prop?
Starting with the antivent plate even with the botton of the boat you may be able to raise the motor.Raise it to a point just before venting becomes a problem

 

[splitshot]

Re: prop size help

no water it is dry...i am probably going to replace the tach with a new this weekend so we will see....yes trimming the motor helped (raising the motor) i had my father in law in the boat he had the gps and i was trimming and watching the mph on the dash guage it was within 2 mph (guage)...bottom is clean and true..when you say venting what do you mean i am kinda new to boating well i have had boats but this is the first one with tilt and trim...everything is mostly in the back of the boat batteries fuel tank our seats are close to half way so the weight is half way and back towards the motor if that makes a difference......motor has been tuned up and timming checked...throttle is opening all the way....all cylinders are 120 except for one it is about 105 cold been the same sine i got the boat a few months ago i have never tried to test compression with the motor warm....thanks Jason

 

[steelespike]

Re: prop size help

Ventilation is the correct term for what some call "cavitation"(when the prop loses grip and motor revs and boat may actually slow.The plate just above the prop is the antivent plate.Start with the antivent plate even with the bottom.There are holes in the mounting bracket that allow raising the motor.I've never done it but you can raise the motor by setting the skeg on a block raise the bow just enough to take off the strain loosen the height associated bolts remove those that pin the location leave those loose that allow movement, carefully raise the bow of the boat till the motor is up one hole.Test and repeat as necessary.The idea is to have as little motor in the water as possible without venting becoming a problem.Could net 2 to 3 mph under ideal conditions.Be sure the motor is secure as you do this.
All that weight in the stern may slow hole shot and could could slow speed
you might test by moving a full tank forward or perhaps a person.
Moving some weight forward will surely improve handling making the bow less flighty.Sometimes just moving midway helps.Not too much weight as it may make the boat lay down and lose speed.

 

[Silvertip]

Re: prop size help

With RPMs off that far I would wonder if the engine is making full power. Many folks assume that just because the engine is running, it is running well when in fact it may have one or more cylinders that are not firing or making power. So before you spend money on tachs and props, do some engine tuning and diagnostics to make sure the engine is performing as it should. A check of spark plug condition after a long run at WOT will tell you if spark or fuel are an issue.

 

[splitshot]

Re: prop size help

just curious what is the rpm i should be shooting for ?? its a 1977 johnson 140 i am probably taking it out this weekend with just me no gear and i'll see what mph and rpm i have with less weight because the motor has been tuned up...thanks