Slow cruising prop

John Sakal

Recruit
Joined
Apr 9, 2017
Messages
2
Hi Boat Guru,
My wife and I are in our seventies and no longer enjoy high speed bumpy planing and the unbearable engine roar. Eileen and I wish to do low speed calm cruising like in canals. We have twenty eight foot Searay Suncruiser with a 350 CID Mercruiser rebuilt last year. We are currently running a four blade 5x16 aluminum prop that gives us good plane and WOT rpm. What prop may we use to give us the best mileage at 1000 rpm. I am not concerned about reducing the Rpm at WOT because we will not be running wide open making those wild dashes to beat the storm or other fishers to a hot spot. Would a 16 x say 20 or 22 be suitable for our needs? Thanks for reading this. I remain in boating
Your friend
John Sakal
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
Joined
Jul 23, 2011
Messages
47,297
first, welcome aboard

at 1000 RPM, the boat is starting to try to come out of the hole. this is the absolute worst speed you have attempt to keep a hull for economy.

back off the RPM to just a tick above idle, or get the boat on plane. with your 28', planing speed is about 20mph and idle is about 5mph

switching from a 20p or 22p wont change your economy at such a low speed since wind and current would have more effect than prop pitch. however making such a large jump in pitch will kill your ability for the boat to come up out of the hole if you need to get on plane for reasons such as an emergency, etc. and will lug the motor reducing its life.

if you want economy in a boat, get one with sails

I say, leave the prop alone, enjoy retirement just putting around.
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
Yes, plainly in agreement with SD, if OB is running at near or max wot rpm range as currently loaded stay with that prop. Hole shot is something to count with when combo needs it. With 1 K revs will achieve just fast displacement speed passing from idle and same if bucking from wot. Just throttle to the desire rpm you both are confy with and enjoy boating at less pace.

Happy Boating
 

jimmbo

Supreme Mariner
Joined
May 24, 2004
Messages
12,944
You still have to prop it so the engine is running in the recommended rpm range at WOT. Plugging any vent holes will propbaly increase the props bite at non-planning speeds, but may reduce acceleration, but on I/Os it might not matter much
 

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,544
Have you thought about trim tabs, adjustable hydraulic at that? Tabs act like you raking your flat hand across the water and then cupping it and doing the same thing. When deployed they force the transom up. This helps in obtaining a faster hole shot when you need it and in keeping the stern up, aka boat on plane when cruising. Adjustable let you put them down when cruising and sucking them up when you want the wind to blow your hair.
 

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,544
You still have to prop it so the engine is running in the recommended rpm range at WOT. Plugging any vent holes will propbaly increase the props bite at non-planning speeds, but may reduce acceleration, but on I/Os it might not matter much

I have seen ported ss props that are listed for OB and IO applications and wondered how well they work as I too have read that ports on IOs aren't that effective. Since IOs are put in the same kind of hulls running OBs just must be the added weight. Don't know.
 

Bondo

Moderator
Staff member
Joined
Apr 17, 2002
Messages
70,465
Would a 16 x say 20 or 22 be suitable for our needs?

Ayuh,.... Welcome Aboard John,..... Ya really oughta stick with the prop ya got, bigger will cause the motor to be Luggin' at all rpm levels,....

That'll lead to detonation, 'n motor melt down eventually,....

A bigger prop is a false sense of economy,...
 

Starcraft5834

Lieutenant Commander
Joined
Jun 2, 2013
Messages
1,677
4 blade prop easier handling as slower speeds, best overall prop in my view, stay within recommended pitch for your engine..
 

QBhoy

Fleet Admiral
Joined
Mar 10, 2016
Messages
8,286
Agree with SD and bondo....
Although...do we think that boat would be trying to plane or coming out the hole at only 1000rpm ? I wouldn't think so ?
Have you thought about selling her and buying a displacement cruiser ? Not sure what's available in the US, but could recommend plenty over here in the UK for equivalent money.
If you are keeping the revs low sustained, might be at risk of filling the sump with unburnt petrol. Plenty of big planing boats with v8's over here been used only on canals at tick over and 5 mph that wouldn't pass a compression test. Not what they are meant for really.
 
Top