18SF planing speed and performance questions

[erebus]

After running the boat a little more frequently after last years rehab, I have a few performance questions, which I thought I'd run by the hive mind.

The boat is a 1975 18' SF.
Last year I put a new 40 gallon fuel tank below decks, all the way forward in the coffin, put the batteries (2X group 24 ) as far forward in the console as possible, and moved the console itself forward from original location about 8 inches. I think the trim is good, as she sits right on her lines, fully loaded.

After removing a ton of rotten wood out of the transom and putting a new coosa core in, and swapping the motor for a 1998 Mercury 115 2 stroke, that came off a 20' seacraft, I was wondering after this past weekends trip across the bay to Wellfleet if the prop etc. should/could be changed/modified.

She doesn't seem to get up on plane until about 13-15 knots and that requires about 3800-4200 rpms.
Once up on plane I can take it back to about 3600 and she'll hum along at about 18-19 knots.
Any less rpms, and she starts to fall off.
Really starts to fall off at about 16 knots.

That's running with the (mostly) stock boat, an AL leaning post, some beach crap, me and the wife (me big, she small) a full tank (40 gallons) 2 anchors, and some miscellaneous tools, oil, boat junk etc.
Oh and a bimini, in the up position. Heres a pic.

Does this all seem in keeping with how she should perform?

Unfortunately the prop is an unmarked aftermarket job, but I'm guessing it's a 13 3/4 X 15.

Any thoughts?

[Snookerd]

I think you're under propped. I had 2 18 SF's with v4 130's. One had a 17p and one had a 19. They both planed and cruised at lower rpm. What is the level of the water line at the stern? Water logged foam in the stringers?

[erebus]

Originally Posted by Snookerd

What is the level of the water line at the stern? Water logged foam in the stringers?

Its still the factory water line (hasn't been raised any over the years, verified when I sanded the transom down), with the boat just floating, loaded for beach, there's about an inch or two of bottom paint showing above the surface of the water.

I also removed the aft fish box for bilge access while doing the transom job, and the stringers appeared sound at that time.

Also, fuel economy seems OK, I guess? Dunno really.
Burned about 18 gallons going from Sesuit harbor in Dennis to Barnstable Harbor, tootling up the harbor and then back to Sesuit.
Last weekend going to Wellfleet and back (from Sesuit) burned maybe 8? 10?
Kinda vague I know... Not sure of the mileages either...

[Snookerd]

Sounds like the prop

[FishStretcher]

Do you have a fin? I can plane at/ just under 10 knots on a 20 MA with one. And a 4 blade stern lifter prop.

And check your mileage figures. Those trips aren't so far, that first one- 18 gallons seems way too high.

[Bigshrimpin]

I have 2 pontoon boat props for that motor . . . . 11P and 13P. Come on over and you can have them both

I'd cross cape cod bay from sandwich and fish ptown in my 18 (when I had it) with a 175hp merc and I burn 17 to 18gallons for the day.

[McGillicuddy]

Yeah, you may have a random bow lifting prop so youre pushing water til it hops up. Go down to your local shop and borrow a or Black max or a QS Black diamond 13-1/2 x 17. Maybe that'll help. I think those motors were known as 2+2s they idle around on 2 cylinders until you goose them, and all 4 kick in. Unfortunately there just is slow or gooo and not much in between.

[GameOnSalmon]

Originally Posted by McGillicuddy

Yeah, you may have a random bow lifting prop so youre pushing water til it hops up. Go down to your local shop and borrow a or Black max or a QS Black diamond 13-1/2 x 17. Maybe that'll help. I think those motors were known as 2+2s they idle around on 2 cylinders until you goose them, and all 4 kick in. Unfortunately there just is slow or gooo and not much in between.

Mcgillicuddy is exactly right... you got to hammer the 2 + 2 motors... I would definately look into the prop selection... I am guessing a 13 3/4 x 17 Stainless enertia gonna fit that set up about right... Depending on the Prop Design you end up choosing...

Option # 2 give the boys over at Propgods a ring and see what they suggest... too much information is not a bad thing.

Robert

[FishStretcher]

I have a free M case prop if you want. Came off a yamaha 175. Splines should line up, not sure on hub diameter.

[erebus]

The motor is indeed a 2+2. I've got it tuned pretty well, so the transition from 2 to 4 is "ok". It switches to all 4 cylinders at right around 2250 which is just where you want to be when you're off plane just poking around.
My wife always thinks the motor is failing.

I did some research and found what seems to be a way to determine pitch on an unmarked prop. Its not 100% mathematically accurate but it got me in the ballpark.
Using it I ascertained that my unmarked prop appears to be a 13x17.
Heres what I found:

Pitch can be measured as circumference x slope,where circumference = diameter x pi, and slope = blade depth divided by width.

It's easiest to measure the depth and width at the hub - in the picture below I drew a yellow line to represent the depth measurement and the green line is the width.

If you measure at the hub, then use the hub diameter in the formula. The actual prop diameter is twice the distance from blade tip to center of hub, but it's extremely difficult to measure width and depth at the tip.



My prop for some reason had no markings, but I suspect it's a standard size for the motor (13.75 x 17). I measured the the depth/width as approx 4/3, and the O.D. of the hub is about 4, so 4 x pi x 4/3 = 16.8

So maybe my prop is OK for the SF18 and I'm just being fussy, I dunno.
Maybe the no-name prop has poorly designed blades, or maybe the boats overloaded and I need to lay off the french fries.

I'm going to see if I can borrow a 13 3/4 x 17 Black Max and a 13 3/4 x 15 Black Max from Nauset Marine and do some benchmarking.