More Performance Possible?

[carolynrose]

Boat: 1988 Bayliner 2556, Factory listed weight 5360lbs.
Engine: OMC 350cu in 260 HP
Outdrive: Factory Cobra w/GLM Lower Unit
Factory Prop: 15 1/2 x 13 Aluminum by Michigan
Current Prop 14 3/8 x 15 Stainless Prop by Solas
WOT 4200 rpm at approx 24mph on GPS, under ideal conditions
Now the question: The boat doesn't get up on or hold plane under 3600rpms, I feel like I'm pushing the engine to hard all the time or is this normal?
Is there a better prop for this boat?

 

[Capt'n Chris]

Re: More Performance Possible?

Is this something new you're experiencing with your boat? Does your prop appear to be in decent shape? Without a whole lot of other detail, we should both assume that the boat ran properly and was able to plane decently at one time, so what has changed? Water can easily and stealthily intrude the hull and cause all kinds of latent problems causing us to think that the motor or prop may be the culprit.

To answer your question and the feel of "pushing it hard", no, you should not have to do that as the boat/engine was most likely properly matched as new. What is the max rpms that motor is designed to turn? Match your prop to that with the load that you carry and make sure your power trim is being manipulated properly from a 0 trim angle.

 

[hwsiii]

Re: More Performance Possible?

Carolyn, welcome to Iboats prop forum and we will try to help you as much as we can, but there is certain information that is needed in order to help you. Working over the internet and not being able to see the boat or prop the question is about makes it much harder to propose a solution to the correct prop that you need, and most people don?t know the make and model of the prop . When we get information in bits and pieces we continually have to look back and forth to try and piece the information together. The following form has all of the information that is needed to help you with your problems in finding the right prop. If you do not know all of the information that is asked for on the form please give us the most information you have available as I have a program I designed to find the right prop for any planing hull boat that needs this information. It requires time to find all this information when we have to look for it and in my defense I expect you to be willing to expend as much effort in finding the right prop as I do. We hope to be able to help you get the best prop we can for your particular uses, and this information is required to do that. It is not as simple to pick the right prop as most people coming to this forum think it is, that is why we ask so many questions, it is for your benefit we ask all these questions.
There are other people who can help you that don?t need this much information, if you would rather wait for them just don?t fill out the information I am requesting. And if you have a stainless steel prop or are changing to one the blade geometry of the prop can make a lot of difference in speed, RPM and the handling characteristics of the boat. Most of my calculations are done assuming an aluminum 3 bladed prop will be used, when changing to a SS or 4 blade prop, or changing within SS props we need to discuss what you are trying to accomplish.

Iboats Boat and Motor Info
1. Year, make and model of boat
2. Length, width and base weight of boat, look for boat decal on back of boat
2a.What is the recommended HP for your boat
3. Number of people and gallons of gas normally on boat
4. What do you use the boat for
5. Is it a Deep Vee and if so how many degrees of deadrise
6. Year, make and model of motor
7. HP and gear ratio of motor IMPORTANT
7A If you have a mercruiser and don?t know the ratio get your serial off the outdrive and go here http://www.sterndrives.com/mercruisersterndrives.html
8. Manufacturer?s recommended Wide Open Throttle (WOT) range
9. Anti-ventilation Plate height above keel of boat if it is an outboard in inches
10. Is it a bass boat or does it have a pad bottom
11. Does it have a hydrafoil, dolefin or trim tabs
12. Make, model, diameter, pitch and whether SS or aluminum prop
13. WOT RPM and speed from your current prop and how much gas and how many people were in the boat for the test data and is the speed by GPS. If you do not have a tach you can buy a Tiny Tach for $ 50
14. Are you at sea level or a higher elevation, give us the elevation in feet
15. Has your motor been tuned up lately and have you checked that the carburetor butterfly is opening all the way, checked compression, and looked at the plugs and checked spark, all of the forgoing could be the reason your prop is not attaining full RPM
16. How long has this prop been on the boat and why, at this time, do you think it is the wrong prop
17. Does the prop show any damage that you can see
18. What problems are you trying to cure or what are you looking for the boat to do that it is not doing the way you think it should or to your expectations

REMEMBER, The numbers I give you will be NO better than the information you give me

The only thing I ask of you is to come back and give me a report of WOT RPM and speed for my database.

H

 

[wca_tim]

Re: More Performance Possible?

detailed analysis aside and assuming that you're not dragging a dirty hull (obvious biology on the hull creates tremendous drag), that's a heck of a lot of boat to push with a single 350. Again, without knowing every detail, I'm betting that the original 13" pitch, 15" diameter prop was a lot closer to what you need.

first lower pitch gets your rpm range where it needs to be for max horsepower to be at max rpms and to run the engine in the propper operating range.

second, the larger diameter is probably important because you're pushing a very heavy boat with a relatively small amount of horsepower. The small diameter and corespondingly small blade surface area doesn't give the prop enough bite on the water and lets it slip (think really skiny rear tires on a pickup - light rear end - with a lot of power).

You're better off by far running the engine at higher rpms and being in the recommended operating range than you are to overprop and run at lower rpms. Lugging the engine - that is overworking it by having it over propped - will wear the engine out far more quickly than running it at 4000 rpms all day long under a lighter load.


Hope this is helpful...

 

[carolynrose]

Re: More Performance Possible?

The boat was purchased last year and originally had a 15 x 15 stainless prop that was in very poor condition. Performance was actually worse than with the standard 15 x 13 aluminum prop. Understanding that stainless props give better performance all things being equal I sought out a prop replacement.

The catalogs I went through showed this as the replacement for the original size. It is actually better than the aluminum. But I am still experimenting and looking for advice.

Listed operating range for the combination I have is 4200 - 4400 at full throttle. My concern is that it needs to run above 3600 to maintain plane, and takes a very long time to get there.

Would I be better off with the 15 x 13 in SS?

We normally have three adults with at least 1/2 a tank and misc fishing gear.

The boat runs very well and this is not a new issue but just looking to maximize what I have.

A 1988 Bayliner 2556 is a 25ft x 8.5ft flybridge boat with a planing hull. Standard equipment on this boat was a 260hp (350) chevy engine with a cobra outdrive. I believe somewhere around a 1.5 gear ratio. Original equipment lists the prop at 15 1/2 x 13 aluminum (mine is a Michigan). WOT range 4200 - 4400. I am and have been running 4200 at about 24 mph (on gps) with a 14 3/8 x 15 Solas SS prop. The boat works very hard to get on plane and finally pops up about 3600 - 3700rpms at around 14 mph and then it goes very well to about 20mph at 3900 - 4000rpms. I try to keep it down to about 3700-3800 and it will stay on plane anywhere from 17mph to 19mph depending upon sea conditions.

It seems like a struggle and a narrow planing band with this prop. Suggestions?

 

[billy4hp]

Re: More Performance Possible?

The boat was purchased last year and originally had a 15 x 15 stainless prop that was in very poor condition. Performance was actually worse than with the standard 15 x 13 aluminum prop. Understanding that stainless props give better performance all things being equal I sought out a prop replacement.

The catalogs I went through showed this as the replacement for the original size. It is actually better than the aluminum. But I am still experimenting and looking for advice.

Listed operating range for the combination I have is 4200 - 4400 at full throttle. My concern is that it needs to run above 3600 to maintain plane, and takes a very long time to get there.

Would I be better off with the 15 x 13 in SS?

We normally have three adults with at least 1/2 a tank and misc fishing gear.

The boat runs very well and this is not a new issue but just looking to maximize what I have.

A 1988 Bayliner 2556 is a 25ft x 8.5ft flybridge boat with a planing hull. Standard equipment on this boat was a 260hp (350) chevy engine with a cobra outdrive. I believe somewhere around a 1.5 gear ratio. Original equipment lists the prop at 15 1/2 x 13 aluminum (mine is a Michigan). WOT range 4200 - 4400. I am and have been running 4200 at about 24 mph (on gps) with a 14 3/8 x 15 Solas SS prop. The boat works very hard to get on plane and finally pops up about 3600 - 3700rpms at around 14 mph and then it goes very well to about 20mph at 3900 - 4000rpms. I try to keep it down to about 3700-3800 and it will stay on plane anywhere from 17mph to 19mph depending upon sea conditions.

It seems like a struggle and a narrow planing band with this prop. Suggestions?

There are a ton of similar boats up here, all of them have massive Bennett trim tabs to keep them on plane and help with fuel economy as boating around here is destination oriented and with rough water conditions you need to keep your speed down... So do you have trim tabs?

 

[carolynrose]

Re: More Performance Possible?

Yes I have Bennet factory equipped trim tabs and outdrive tilt. I have played with these things all over the place and the infromation I am giving is maximized as best I can with them.

 

[billy4hp]

Re: More Performance Possible?

Yes I have Bennet factory equipped trim tabs and outdrive tilt. I have played with these things all over the place and the infromation I am giving is maximized as best I can with them.

What size are your tabs? Alot of factory equipped tabs are too small to be useful....

Take the time and fill out the form hwsiii posted earlier. He worked wonders on my boat by having me go to a 4 blade stern lifting prop....

Good luck...

 

[carolynrose]

Re: More Performance Possible?

Response. I forgot to mention that this boat is kept in a slip in Cape May, NJ. Most of the time used for fishing the Delaware Bay in seas from 1 - 3 ft, any more than that we don't go out. Not sure of the trim tab size because the boat is in a slip not near my home. But if I had to guess the tab size looks to be about 12" wide by about 10" deep with 1" lips.

If I missed any information let me know and I'll try to provide it the best I can.

 

[45Auto]

Re: More Performance Possible?

Carolynrose, I think that what you are seeing may be typical performance for your boat. I have a friend with a 26' Searay cruiser, about the same weight as yours, with a single 350/300 HP Mercruiser. It is very sluggish to get on plane, and cruises at about 25 MPH at 4100 RPM. I've never tried, but I don't think it would hold plane at 3600 RPM. I would be surprised if you see much difference no matter what you do with your prop.

 

[hwsiii]

Re: More Performance Possible?

Carolyn, the reason I ask all the questions I do is because my program requires exact numbers to correlate information for finding a better prop. I have a hard time believing that the gear ratio of your outdrive is 1.5:1, because if that is true you are showing prop slip numbers at wide open throttle of over 40% and that is just exceedingly high, but there is a possibility the prop you are using could be that far from the correct blade geometry you need. Here are my numbers showing your present prop slip with the numbers you gave me.

Prop Slip

1. Have you checked the bottom of the boat for a buildup of paint or marine organisms?
2. If you are not using a bottom paint that is vinyl based and it is flaking you need to change that for next year, you can't afford any extra drag created from paint sloughing off and creating a rough bottom. If you have that problem the bottom needs to be sanded good and repainted with the vinyl paint or a self polishing paint.
3. You have to have a 4 bladed stern lifting prop for this boat, it weighs too much for the horsepower that you have available for it. Your power to weight ratio is over 25 pounds per horsepower with only 1 person on the boat and fuel with the Coast Guard requirements.
4. My information for your boat shows a different gear ratio and different RPM than what you show, but mine could definitely be wrong? Please verify your information.
5. The trim tabs will not hold the stern of your boat up, even if you get larger ones, they just help change the attitude of the boat when running for a better ride. If you get larger ones to try and hold the stern up when you deploy them in the down position they are just going to create more drag, which you can't afford.
6.

The boat was purchased last year and originally had a 15 x 15 stainless prop that was in very poor condition. Performance was actually worse than with the standard 15 x 13 aluminum prop.

The catalogs I went through showed this as the replacement for the original size. It is actually better than the aluminum.

These are conflicting statements and it makes it harder to understand what is really going on with the two different props!!!!
7. I need to know exactly what make and model of SS prop you are now running on the boat, it will have numbers on the side of the prop or underneath the prop nut or the back of the prop.
8. In my opinion with the information you have given me so far is you need a 13" pitch 4 blade zero rake prop to be able to lift the stern or the whole boat out of the water and to be able to still reach your recommended WOT operating RPM, It doesn't matter whether it is aluminum or SS, and the aluminum is cheaper.
I have been trying to find one in aluminum, but so far i have had no luck, I will look further though. And if I have no luck you will have to buy a SS.
Please answer the questions with as much detail as possible. And i expect you to see an increase in speed as well.
45Auto, the correct prop will make a lot of difference, in my opinion.

H

 

[45Auto]

Re: More Performance Possible?

My experience has been that if a big, heavy, relatively underpowered boat, is propped so that the engine is turning in the recommended RPM range then you aren't going to see anything from diddling with the prop. The power-to-weight ratio is so low that the drag and weight of the boat far overshadow any minuscule gains you can make with the prop.

You've got a 26', 5300 lb boat plus probably at least 500 lb of gas, 500 lb of people, and several hundred pounds of gear. I wouldn't expect a single 260 HP (when new) engine to do any more than what you're seeing now no matter how much you spend on props.

 

[carolynrose]

Re: More Performance Possible?

OK, lets see if I can sort this out better. The upper gearcase ratio listed for this model is 21:16. The lower gear case I bought from GLM and is listed at 14:26.
Memory is a tricky thing so I pulled my file and the prop I am currently running is this.14.375L15 3BL SS APOLLO MWC 99305214.375L15 3BL SS APOLLO MWC 993052. 14.375L15 3BL SS APOLLO MWC 993052. Manufacturer: MICHIGAN WHEEL Item Number: MWC-993052.
As I said the original prop was SS and in horrible shape so I replaced it with a Michigan 15.25 x 15 prop. I only used this prop until the SS prop came in from the Marina I ordered it from.
I have the original prop that came with the boat new. It too is in bad shape but it is a 15 1/2 x 13 aluminum prop(make unknown) and never used it.
There is no issues with the way the engine runs, I have been a Automotive Master Mechanic for 30 years and have it tuned to run and shift perfectly. I am no marine mechanic but I love to tinker with things to make them right almost to an obsession.
I think I am missing the mark with the prop. But I would like the boat to get on plane without so much struggle and stay there comfortably. High speed is not a concern, 20 -25 mph is fine. The service manual lists the WOT at 4200 - 4400. I can hitr 4200 at 24 mph.
Bottom growth is not an issue at this time and we just sanded and bottom painted before we went in for the season. I am aware that this can slow down the boat. But at this time the performance seems about the same as when we went in.

 

[hwsiii]

Re: More Performance Possible?

You would have had to work real hard to pick a prop less desirable for your boat, sorry. I run that same apollo prop on my flats boat, it is a high rake prop which is trying to raise the bow of your boat, thus pushing the stern down and creating even more drag than you would have normally and it is creating a high bow condition making the boat run at the wrong attitude in the water as it is cruising and you are having to use the trim tabs to try and raise the stern which is just creating more drag and prop slip. That is why it requires so many RPM to try and keep it on plane and it keeps wanting to fall off plane at lower RPM and speed.
The marina should have told you that when you ordered it, that prop could potentially work if you had twin engines, but it still would not be the ideal prop.


H

 

[hwsiii]

Re: More Performance Possible?

I forgot to tell you I come up with a 1.41:1 gear ratio for your outdrive.


H

 

[carolynrose]

Re: More Performance Possible?

Makes sense. What would you recommend?

 

[hwsiii]

Re: More Performance Possible?

My recommendation is a 13" pitch prop with zero rake to raise the stern, the problem I am having is all the props that I am sure will work will not fit your outdrive, or are not small enough pitch.


H

 

[carolynrose]

Re: More Performance Possible?

Here is the list of the props I have at home.
Vortex Michigan Wheel 992002 15.25 x 15 (new, about 2 trips)
17926 15 x 15 SS OMC/Michigan (very bad shape)
391536 15.5 x 13 OMC/Michigan (rough shape, previous owner said it was the original) if these help with any numbers your looking at, they all fit, the Vortex is a interchangeable hub. If the original is the best match, I can just buy a new one

 

[hwsiii]

Re: More Performance Possible?

The Mihigan wheel 15 x 13" will work, but it will not carry the load of the boat very well, and the RPM should be too high. You need a 4 blade with zero rake for optimum performance and you will really like the boat with the 4 blade and the fourth blade will reduce the RPM to the right level.


H

 

[carolynrose]

Re: More Performance Possible?

Recomendation?