3.0 Mercruiser Prop Question

[Crazydave1243]

Hello to you'll I have a 1985 thunder craft I?m not sure of the exact model the only number and the plate is model 162. The boat has a 3.0-mercruiser and alpha one gen one out drive not sure of the gear ratio. From the information that I have found on iboats the boat weights around 2000 pound empty, with gear and fuel, myself and everything I?m guessing the total weight is around 3300 pounds give or take. The boat is 17 foot long and is balanced well right now am running a Michigan wheel 14x19 prop which is aluminum. According to my tach I?m turning around 5000 rpm at 38 mph according to gps when I trim my motor up I do get some blow out. The boat gets on plane in no time; I?m not looking to pull a tube I want more speed. I never have over 4 people on board and we just cruise around. Thanks in advance for all the help.

 

[Bifflefan]

Re: 3.0 Mercruiser Prop Question

You can try a 4 blade 20 or 21p.
130 HP on a 17' is a good power to weight ratio.
The 4 blade should help with the blow out, and as for the trimming up, If it blows out, dont go that far.
I only use a minimum amount of trim if any.
Do have trim tabs? Or a whale tail? If you have a tail on it, try taking it off. If you have tabs use a little less or if they are smart tabs, move the piston to a hole that makes less lift.
But I would go with the 4 blade. Once I tried one I will never, not for any reason go back to a 3 blade.

 

[QC]

Re: 3.0 Mercruiser Prop Question

Nice chunk of info/data. Thanks.

Your numbers look about right actually, I am using a 1.98 gear ratio. I end up at 16% slip. That, and your blowout comment, tells me we can gain you some speed.

I think before I did anything else I would try shifting some weight aft. You mentioned blowout at higher trim levels and that is typical. What we really want is your bow more out of the water with higher trim levels and minimal porpoising. Will she porpoise now? Or does she begin to blow out first? If she won't porpoise, I'd do a weight reduction and rebalancing exercise:

1) Pull everything out of the boat except seats and fuel

2) Get rid of anything you don't really need for a typical day and/or safety.

3) Re-stow everything by placing the lightest things towards the bow and the heaviest things towards the stern. Any shifting this way should help with the blowout and subsequently some top speed improvement.

4) If she starts to porpoise before she blows out, shift some weight back forward until you get a good balance compromise.

5) Note RPM and consider prop changes then

Good luck!!!

Edit: Sorry BiffleBoy, we were posting at the same time. I think both posts are valid as long as the tach is right.

 

[Crazydave1243]

Re: 3.0 Mercruiser Prop Question

Thanks for the replies and do have an Atwood whale tail on it, and I have yet to verify my tach. I have heard that a four blade stainless would help but I would loss some on the top end. There isn?t any shop here local that will let you "test and tune" with props. I have also been told smart tabs would help to. This is the first boat of this type I have owned so I?m new to the inboard scene so any help is dearly appreciated.

 

[Bifflefan]

Re: 3.0 Mercruiser Prop Question

Like QC said, do the weight stuff first.
Tabs will only help get you on plain, and some control issues.
What you are getting to much stern lift causing the prop to loose grip on the water. Kind of like hitting a slick spot on the road when your driving and the tires spin.
You mentioned 5000 RPM. Is that with the throttle slammed wide open?
If you have more throttle left but are only turning it at 5000 then you may be able to use more prop.
Remember that you only have a 4 cylinder engine, so the more weight you lose the faster you can go.
Just for the cost, i would shy away from the SS props. Alum is fine for your boat and the performance difference wont even be noticed. Thats my opinion. Others may say different.

 

[Bifflefan]

Re: 3.0 Mercruiser Prop Question

Edit: Sorry BiffleBoy, we were posting at the same time. I think both posts are valid as long as the tach is right.

Nope, your right on. I didnt go in to it as deep as you did and only touched on some things I thought would help.
I think we can get this issue fixed up just fine.

 

[Crazydave1243]

Re: 3.0 Mercruiser Prop Question

Ok I will do that thank you'll for the replies and I will repost my findings

 

[JimS123]

Re: 3.0 Mercruiser Prop Question

If your only objective is more speed, go with a 3-blade SS double cupped 21" prop. A 4-blade may provide some other benefits, but top speed will suffer.

 

[dannieboiz]

Re: 3.0 Mercruiser Prop Question

I would add hydro foil to help with the plane, I had an 19.5 Capri 02 3.0 on an Alpha Gen II with almost the same exact weight as your boat with a 4 blade 21p and saw maybe -1 mph difference in top speed from the 3 blade. I was doing 45mph all day long. it's just really nice at the dock when you can maneuver around at almost no throttle and just in gear.