Excessive slip

[Southtowns27]

Boat is a 1986 Wellcraft 260 Aft Cabin. Engine is a Chevy 350, 260 horse, Alpha 1. Prop is 16x16. Gear ratio is 1.5:1. WOT I get 4400 RPM and 31 MPH by gps. When I plug in all the numbers, I come up with 30% slip. Ugh. I guess I now know why my fuel economy is so abysmal. According to my WOT RPM I'm right on the money, but I need something that has more bite. I know the prop is not factory correct, but it was on the boat when I got it. Not sure what a previous owner was trying to accomplish. What prop should I be looking at for my application?

 

[Bondo]

Re: Excessive slip

Ayuh,.... How long has she been sittin' in her slip, before yer tests,..??

 

[Southtowns27]

Re: Excessive slip

I trailer the boat...

 

[steelespike]

Re: Excessive slip

At wot your 260 would use about 26 gph.Do you know your actual gph at wot? Have you attempted to find your ideal cruising a speed?
Cruising speed could be anywhere from just on plane to about 1000 rpm higher.
Do you have trim tabs? It has been shown that while tabs can help a boat stay on plane easier and handle better they can use more fuel,especially if used a too aggressively. While your rpm seems reasonable you might want to confirm it before any changes.

 

[Southtowns27]

Re: Excessive slip

I don't run the boat for extended periods at WOT. I get it up on plane, then pull back and cruise at about 25 mph, which is about 3700 RPM. I haven't been able to figure out GPH perfectly since I never fill the gas tank all the way. Near as I can tell, at this speed I'm around 18 gph. That seems way high, and it also seems like I should be going faster than 25 mph at 3700 rpm. The boat does have trim tabs, but I hardly ever need to use them. The boat comes up on plane easily and stays there until I get below 17 MPH.

 

[steelespike]

Re: Excessive slip

I believe the boat is 5000 lbs dry. A heavy boat will generate higher slip numbers.
Usually as rpm goes down slip goes up. At 3700 your slip appears to be about 33%.
Do you run 3700 rpm because it feels right or because it seems to produce the best mpg?
25 at 3700 seems reasonable. 18 gph delivers about 1.40 mpg.
Is the prop aluminum?
If the prop appears like it has been on there a long time suffering some wear and dings it may need replacing.
If its from 85 it may be somewhat primitive design.

 

[QC]

Re: Excessive slip

Agree with above. It may also be that the prop has been reworked to a lower pitch. Pure speculation, but the fact that she planes easily makes me wonder. 4400 is not a high WOT RPM which is what I would expect with something that planes easily. If it was acting as a 14 pitch I could see an easy plane and your slip percentage would be "better". Heavy boats don't typically plane easily with a single small block and single prop drive. My .02. You need to test with something else. Also the drive ratio could be wrong.

I would verify the tach as well, but that actually seems right to me as it would have to read higher to make sense which would be an even worse slip number.

 

[jestor68]

Re: Excessive slip

Try using a 15 X 17 with that gear ratio.

 

[Southtowns27]

Re: Excessive slip

I cruise around 3700 because it seems happy there. I've tried slower and faster, but it just seems to settle in the best at that speed for some reason. The prop is aluminum and did have a couple small dings in it when I got the boat. I brought the prop to a local prop shop and had it redone. The prop guy confirmed that it is a 16x16. Do you think it's possible that because the boat is so big and heavy that the prop is flattening out under load? I don't know the make of the prop, but it could very well be a cheap one. I guess too when I say the boat comes right up on plane, it doesn't just jump out of the water. I hear the secondaries open and it works really hard for a good 30 seconds before she's up to speed. Once there, I pull back to about 3700 and it feels like it would stay there for weeks..
30+% just seemed like a really high slip number, but for a big heavy boat, maybe not. It also seemed like the boat should run faster than 31mph, but again, maybe not. My buddy has a Baja with the identical drivetrain in it. He has 15x17 and 15x19 props that I'm sure I can borrow. Might be time to do some testing.
I guess I was hoping that I would get an answer like "try using prop X", but I guess I'll throw things at the wall and see what sticks

 

[QC]

Re: Excessive slip

Heavy or not AL does "flatten out" (flex). Throwing stuff at it is pretty much how you do props.

 

[Southtowns27]

Re: Excessive slip

Hmm.. I wonder if a stainless prop would do anything for me

 

[steelespike]

Re: Excessive slip

The 15 X 17 may be a good test prop. Probably not right but will give numbers to work with.
Usually a heavy boat will do better with a larger diameter.But again a good test.
The 16 X 16 prop seems to pop up now and then usually on a heavy boat.

 

[steelespike]

Re: Excessive slip

The 15 X 17 may be a good test prop. Probably not right but will give numbers to work with.
Usually a heavy boat will do better with a larger diameter.But again a good test.
The 16 X 16 prop seems to pop up now and then usually on a heavy boat.
A Solas Amita or Rubex in a 15" 4 blade may help. They use "squeeze cast" technology,Thinner stronger blades.
May improve on your already good cruise performance.
4 blades are said to improve holeshot,stay on plane easier,run smoother and respond better around the dock.Being a 4 blade it should help with the slip.

 

[Southtowns27]

Re: Excessive slip

I thought 4 blades were more inefficient though. I'm not terribly worried about my holeshot. More responsive during maneuvering would be nice, this thing handles like a wet sponge. I guess my ultimate goal in reducing the slip is to maybe pick up a few cruise MPH and some fuel economy. I'll do some research on the props you mentioned

 

[smokeonthewater]

Re: Excessive slip

you aren't gonna gain much if anything in the mpg area you are pretty close... you MAY need to slow down just a little at cruise 15x17 will be way too much prop for you as the 16 is all she wants... the 15" diameter will INCREASE slip at lower rpm's... if anything you might try a 16x15.... on your other thread I said I had a 16x16 but I believe it was actually a 16x15

forget slip numbers and prop for hull design and rpm... IE heavy so more surface area (diameter and number of blades) and look for 4400 or even 4600 WOT

IMHO, you will never see anywhere near 2 mpg with that hull on plane...

 

[Southtowns27]

Re: Excessive slip

Good info. I wasn't expecting a silver bullet. Better economy would be nice, but it is what it is.
What's your opinion of 4 blade props?

 

[smokeonthewater]

Re: Excessive slip

like em... our boats have some performance similarities but are a bit different too... mine with twin 260's is more powerful for it's size but is more stern heavy.... I plan to run a couple of 4 blade props for more stern lift but haven't done it yet... you probably don't need the stern lift as much as I do but you need more 'out of the hole' help than I do... I think you are a little overpropped as is so I'd either drop to a 15 just like you have or consider a 4 blade but I'm not too sure about whether to try a 15 or look for a little lower in a 4 blade

 

[26aftcab454]

Re: Excessive slip

16 x 15 - 4 blade aluminum might be the ticket-- a good local propshop might loan and exhange so try them out before you buy. I does not hurt to ask.- I had the 454 with BravoIII and got around 1 mpg but I cruised 28mph @ 3800 rpm everywhere. One of the reasons I sold the boat. Never turned on the fridge, stove , microwave, hot water , or A/C . my buds teased me saying I had a 26 ft runabout so I sold it and got a neat little runabout.