Marlin propeller selection

[Spearfish grady]

I have a 2003 Marlin just installed 2 new Suzuki 300 4 stroke outboard motors. They cavitation beginning at 2600 rpm’s I have tried 15X21.5 3 an 4 blade props. Still have cavitation. My mechanic wants to go from 25 inch to 30 inch motors. I’ve read a lot and it may be the wrong pitch prop. Ant ideas

 

[Harrysea]

I have the same motors. I use 16 x 17 pitch Suzuki props to get to 6,000 - 6,100 RPM. With a tower and bottom paint I needed the lower pitch to get to the top of the RPM range lightly loaded. I’m not sure why you’d want 30” shafts. What mounting hole are you using?

 

[Spearfish grady]

Thanks for your reply. I’m as low as possible on mounting holes. I was told by Propgods Ken to to install a 17 degree pitch which I’m now in the prOdessa of acquiring. My problem is cavitation or ventilation starting at 2500 rpm. I am pitched with 21.5 now. I won’t let the mechanic who sold me the engines to do a 30 inch. I’m hoping the 17 pitch props will solve the problem.

 

[seasick]

I have a 2003 Marlin just installed 2 new Suzuki 300 4 stroke outboard motors. They cavitation beginning at 2600 rpm’s I have tried 15X21.5 3 an 4 blade props. Still have cavitation. My mechanic wants to go from 25 inch to 30 inch motors. I’ve read a lot and it may be the wrong pitch prop. Ant ideas

The fact that you feel ventilation at a specific rev range may be a clue as to what is going on. I would guess that at that rev speed the hull is stratin to get on plane and as it does, the props will be higher as the bow lowers and the hull lifts up. If the props and ventilation plates are too high, the props will ventilate. You need to look at the ventilation plate when at that speed and see where it is relative to the water line as well as the keel.
Also note that the Zukes have different gear ratios than a lot of Yamis so you cant assume that a certain diameter/pitch prop that works on a Yami powered hull will work on your situation.

I would also like to know what you mean by cavitate or ventilate at 2600 revs. Do you hear the prop breaking the surface? Do the revs is throttle applies shoot up fast but the boat doesn't react and gain speed.
What shaft length was on the hull originally? If 30 inch, your zukes should be the same,

 

[Spearfish grady]

Great input thank you. The previous motors were yamaha 225’s. Now Suzuki 300 due mostly that I was waiting a year for new Yamahas they are not available.
‘yes you hear engine revs go up and see on display they go up from 2500 to 3200
the old Yamahas were 25 inch and these suzuki’s are 25.
they came with 21.5 pitch 3 blade props. The mechanic then said we need 21.5 4 blade props this did not help.
I’ve had several people tell me to go witch 17 inch pitch which I’m currently trying to order from Ken at propgod.
also got a message from someone with same boat and same motors who has a 17 pitch so I’m hoping I’m on the right path
cavitate or ventilate confusing at best . But I do not hear that sucking sound of props sucking air from surface. All indications are they are ventilating and are deep enough
much has been said about not going to a 30 inch which has to do with torque increasing on transom
thank you for your discussion any mor input would be appreciated.

 

[Harrysea]

The largest Suzuki 16" diameter prop you should need is 18.5 pitch. I have a tower and bottom paint and needed the 17 pitch prop to get my RPM up to 6000. The proper hole for a Marlin with new Suzukis is second from the top. You shouldn't have a cavitation problem.

 

[SkunkBoat]

‘yes you hear engine revs go up and see on display they go up from 2500 to 3200
. But I do not hear that sucking sound of props sucking air from surface.

When you are testing..
Are the motors trimmed IN all the way when you are at a stop and are the trim tabs UP (doing nothing)?
You should accelerate from a stop with motors tucked in and no trim tabs, then trim the motors out as you come on plane.
If you accelerate with motors part way out you may over-rev/suck air.

Are you sure that your motors are set to allow you to tuck them all the way?
Years ago on this forum someone said that their Suzukis were set to stop at vertical and had to be reset to tuck all the way in. IDK if that is true but it was said. My 200 zukes will tuck all the way in and I have to tuck them to take off.
When they are tucked, it jumps up on plane quickly and then I slowly trim them out as I accelerate.

IMO test your props with no trim tab. Make absolutely sure they are all the way up and then don't touch them. They just add a bad variable to the testing. Your boat should work fine using the motor trim and no trim tabs.

What diameter are the props? 16 dia x 21.5 pitch x 4 blade props are really "big" and I can't imagine they could ever over-rev if they are in the water. I would think they would be hard pressed to get you up to the top rpm range.
Proof of that is that Harrysea needs 16x17x 3 blade props to reach 6000rpm
Suzuki tends to go with 16" props because the motor is geared lower and able to put more torque to the prop

Suzuki makes 15.25 diameter 4 blade props. Is that what you tried?

By adding a blade you generally have to go down in pitch and/or diameter. The point of 4 blades is to have 3 blades deep when one is near the surface vs 2 blades deep.
Going down from 16 dia to 15.25 also helps by making the top of the prop lower by 3/8". They are less prone to grab air in turns and in rough seas.


As for 30" shafts. JUST SAY NO! Tell them to get the right props! (Try the 3 x16x17 like Harrysea)

 

[Spearfish grady]

Hope to be installing 2 15X17 props today
motors are tucked and no tabes in use. All good out of the hole but rv’s go up from 2400 to 3000 for a second then catch for 30 seconds and keep on doing it
ill post results of change from 4 blade 16 inch 21.5 pitch too 3 blade 16 inch 17 pitch later
i agree we are not going to a 30 inch motor.
I only use trim tabs once on plane to level the boat side to side. Bow rides on plane with minor if any tab use.