need help

[boatguy19]

i just bought a 86 245 fourwinns sundowner with a merc 350 260 hp. my wot is 3800 rpm, it should be around between 4200 to 4600 rpm the motor is strulging at that rpm and i dont want to blow it up. and my speed is a wopping 20 to 22 mile an hour.

im not looking to go fast but i think it should do a little better than that. with twins it is suppose to do about 48 but i have a single so anyway the prop i have on it now is a 15.50 x 15 pitch i was told to go to a 16 x 13 pitch which they said should bring the rpms up to about 4400, if i do that will i lose speed or no. it takes forever to get on plane if i can even get it on plane with trim tabs down. the boat weights 5000lb without all the junk on the boat.

i would like to get this right the first time as i dont have the money to buy a bunch of props, the motor is only a couple of years old and runs really strong i dont have a problem with the engine at all. i did call fourwinns and they still had records on it and said the 15.50x15 was what came on it but the rpms just dont seem right to me at all. thanks for your help in advance and sorry for the long post.

 

[agallant80]

Re: need help

Yeah, that is not right. The prop may be causing issues but have you looked at the motor? That sounds really off.

 

[dazk14]

Re: need help

Borrow a timing light with adjustable advance. Check total timing at 3,600 rpm.

 

[Sea Rider]

Re: need help

Is 3800 achieved at wot with lightly loaded boat and with you alone ? If not, re test and checked max rpm at full throttle, with a 13 pitch theoretically will gain somewhere 300 to 400 + rpm, being optimistic, still on the poor side of the moon, ideal would be 4400, just between 4200-4600 rpm wot safe parameters. If that boat is rated for 2 engines, with one probably underpowered with worst scenario being already under pitched. Probably a 4 blade prop correctly pitched will work more properly ?

Happy Boating

 

[boatguy19]

Re: need help

does anyone know what the timing should be on that motor.

 

[boatguy19]

Re: need help

3800 rpm is max with two people and a cooler. how much more lower could i go below a 13 pitch to get it close to 4600 rpm. the motor sounds great at 3800 but its just to much weight for a 15 pitch prop.

 

[dazk14]

Re: need help

does anyone know what the timing should be on that motor.

Since the motor has been changed, I can't speak to what you have in your boat.

I don't have the manual in front of me, but I recall 8deg initial, + 24degrees from Thunderbolt IVignition module = 32degrees total timing if fully by 3600rpm's.
I won't be back to the shop for a couple days, so post on the Mercruiser forum on this site to double-check.

It is not uncommon to see the total timing off and loss of power as a result. Even when base timing is correct.

If you need to switch props, the 13" should give you plenty of RPM's. Your approaching peak HP, so you'll see more rpm's per drop in prop pitch.