Changing Prop Pitch

[Tarpon828]

I just purchased a new to me boat. It is a 2005 Sundance 18CC. It has a Yamaha F60 4 stroke on it. I took it out yesterday and the WOT on it was only 4800 RPMs. Speed was 27.9 mph according to my GPS. A few other things. 2 people in the boat (Around 400 pounds) Quarter tank of Fuel (Around 8 gallons) Some fishing gear (maybe another 25-30 pounds)

The running Range according to the Yamaha site should be 5000-6000 RPMs. So I know that I need to go to a lower pitched prop. The Current Prop is 10 X 15 - G. What would be a good pitch to drop to? I am thinking 12 or 13 but I am not sure.

Thanks ahead of time for any help

 

[rndn]

Re: Changing Prop Pitch

About 200rpm's for each inch of prop change with all other things equal. It appears to me that your rig may be seriously underpowered. 60hp for an 18 foot boat is really going to have to work hard.

 

[rndn]

Re: Changing Prop Pitch

Your boats dry weight is 1445 pounds. It's too heavy for a 60hp outboard especially a four stroke. With the weight of the motor, fuel, battery(s), people, and stuff you are well over 2000 pounds. You should really look into getting a larger engine. Look at the max horsepower rating for your rig and always try to stay at least 75% of that max. If your rig has a max HP rating of 120HP, then the smallest motor you should try is 90HP. I don't know what your max rating is, the above was just an example.

 

[PBKboater]

Re: Changing Prop Pitch

Tarpon, Just for future refference: www.caravelleboats.com/propeller info.htm

Paul

 

[Tarpon828]

Re: Changing Prop Pitch

Your boats dry weight is 1445 pounds. It's too heavy for a 60hp outboard especially a four stroke. With the weight of the motor, fuel, battery(s), people, and stuff you are well over 2000 pounds. You should really look into getting a larger engine. Look at the max horsepower rating for your rig and always try to stay at least 75% of that max. If your rig has a max HP rating of 120HP, then the smallest motor you should try is 90HP. I don't know what your max rating is, the above was just an example.

The max HP rating on the boat is 90hp. According to the info that I got off the sundance boat site, the recommended 4 stroke hp is 60 for the 18CC. The recommended 2 stroke size was 75hp. I assumed based on this that a 60hp 4 stroke Yamaha would be OK. Perhaps a bad assumption on my part. For this year, I will not be able to repower it. Just not in the budget. I will see what a new prop does for me this season and see about budgeting for a larger outboard maybe for next year.

Thanks to everyone for there help so far.

 

[steelespike]

Re: Changing Prop Pitch

You should optimize your setup before making a prop decision.
raise the motor vertically until just before venting becomes a problem.
an inch increase could get you 2 or 3 mph depending on where you are now.
Once you get setup with careful height and trim, give us the final max rpm and speed and typical load.
As I understand it a 4 stroke needs to be propped for its max rpm.especially in your boat motor combo.trying to get that 1200 rpm with just with a prop would put you at about 9 inches.sounds extreme to me.9 inches is about what you would put on a pontoon.

 

[Tarpon828]

Re: Changing Prop Pitch

I looked at the boat last night and I think I see what you are saying about raising the motor but wanted to be sure. There are 4 mounting holes in the motor bracket that attaches to the transom. It is currently in the second hole down. So to raise the motor, I would need to move the bolts to the 3 hole down and then take it to the lake and try it and see what happens, correct?

Also, I looked at my old boat (seating beside my new boat, so I thought I would compare) The top plate on the lower unit on my old boat is about even with the bottom of the boat. The top plate on my new boat is about an inch above the bottom of the boat. So if I move the motor up, this will raise that plate even farther up, at least I think it will. Now granted, the old boat is a 1975 glastron that has been the family since new. So I am not sure what went into setting it up (my memory is not that good). I basically just want to make sure that I know what I am doing before I start messing around with it. Don't want to mess up.

One last question, by venting, I assume you mean that I hear the prop sucking air. Like when the engine is trimmed up too far. Correct?

Thank you for your help

 

[Silvertip]

Re: Changing Prop Pitch

Something is wrong with the engine power recommendations. If a 75 HP two stroke was recommended, why would they recommend a 60 HP 4-stroke knowing that they are heavier, less hole shot, and "generally" less top end although not much and with 15 HP less. If I were chosing engines based on a difference betwen 2 and 4 stroke, I would have chosen the 75 two stroke and a either a 75 or 90 four stroke, but definitely not a 60.

 

[fishdog4449]

Re: Changing Prop Pitch

thats way underpowered. you should be running at least a 75 or 85 two stroke. fourstrokes have nothing on a twostroke as far as the torque and holeshot
i run an 80 merc inline 4 two stroke on my glassmaster and i couldn't imagine going any lower than that

 

[steelespike]

Re: Changing Prop Pitch

I have to agree with these guys but you have what you have so lets work with it.Be sure you are getting full throttle just because its a new setup doesn't mean all is right.You may be able to raise it one more hole.It may ventilate but you may be able to improve that with a stainless prop.Be sure it pumps water.
If you can't get it to run right up a hole go for a 10 inch stainless and try again.That should put you around 5,800.