Repropping my cabin cruiser.

[rs2k]

I have a question about re-propping my twin engine cabin cruiser.

I have a 28 foot twin Chevy 350 260HP stringer powered cabin cruiser that weighs around 6000 lbs with an RPM limit of 4600 RPM and suggested WOT range of 4200 - 4600 RPM.

Right now I can only get 3850 RPM and 44.5 MPH GPS verified on smooth water with no wind. At full throttle the boat very smoothly gets up on plane in about 4 seconds, but it will not plane at all on a single engine. If I try the back of the boat sits firmly in the hole at 6 - 8 MPH and the engine struggles to maintain 2800 RPM. If I get up to full speed and than reduce one engine to idle the single cannot even keep the boat on plane. Obviously the boat is way over propped for one engine and at least a bit over-propped for two engines.

This boat is a fishing boat. I really don't need to go any faster than 45 MPH. I got a two engine boat so that I don't have to worry about getting caught out in a storm if I loose one engine 20 miles from shore. The problem is if I loose one engine my speed is reduced from 45 MPH to 6 - 8 MPH.

I think the best thing to do would be two find a pair of props that will allow the engines to run together at 4600 RPM. I'm a bit worried about doing this though. I like the current performance of the boat as it is right now. I also don't like the idea of stressing out the old stringer drives by increasing the RPM they run at. (Maybe this worry is not valid). This will also mean I will have to refuel more often. It's going to take more engine power to keep the same trolling speed and I only have a 100 gallon tank.

Since changing the prop could be done fairly easily from the swim deck, I have thought about keeping a spare prop just in case I loose an engine and have to get back in quickly. This doesn't really seem like a good option though. If I loose an engine and need to get back because of a storm than the current ocean state is not likely to allow changing the prop. It also increases the risk of loosing both engines do the clumsily dropping all three props overboard while trying to put them on.


Any thoughts or opinions on what I should do?

 

[junior1113]

Re: Repropping my cabin cruiser.

what size props are you running?

 

[Bifflefan]

Re: Repropping my cabin cruiser.

Well you didnt say what pitch you have, but i really dont think that matters anyway. I too dont run WOT, ever. But I would try dropping down 2"p and i believe that the RPMs may come up and you should be able to run the same speed, as your not topping out your RPM's with the props you have. also this will help the single engine running. I have talked to the local dealer and they have a (limited) selection of props that they lend out (you have to leave a deposit) for testing. there may be something like that around you.
They do make a plastic prop for "emergency use" and floating prop wrenches, but as you say if the weather and the need arise at the same time, its not a good time to be in the water anyway.

 

[rs2k]

Re: Repropping my cabin cruiser.

The prop size slipped my mind. I'm running 14x21 Stainless Steel cupped props. (Came with the boat)


This is part of a project in preparing this boat for Ft Lauderdale, FL. I want to get this old lake boat ready for a whole lotta Florida Fishing. Catfish just don't cut it.

 

[hwsiii]

Re: Repropping my cabin cruiser.

Rs2k, it is normally cheaper to fix drives than it is engines, and as you are loading those engines up at all speeds with that low of RPM and in my opinion I think you are probably only getting about 200 HP each out of those engines after looking at a Power/Torque for a GM 350 Vortec because of the low RPM you are running at. You have the engines lugging at ALL speeds when you are that far below recommended RPM, you are using more fuel because of the extra load on the engines at all times other than idling and putting a lot more load on ALL of the internal engine parts at all times. The boat is a LOT overpropped, even for both engines and your fuel economy is shot, if you had a fuel flow meter you would see that. And it will require LESS fuel for trolling as the lower pitched props will create more low end performance and acceleration, and because the engine is loaded less your fuel use will decrease. Also because of the increase in low end performance you stand a better chance of being able to plane on one engine. And you should suffer only a minor loss in speed because the increase in RPM will make up for the loss by lowering your pitch.

I hope this gives you a different outlook on what is going on with your boat right now versus how it will be by putting the right props on the boat, and I hope you will take it as I intended it to be, just explaining what is going on with your engines right now.

If you would like more information about prop slip and other analysis of what is going on right now with those props give me the information I need and I will run some analysis of the numbers.



H

 

[rs2k]

Re: Repropping my cabin cruiser.

Thank you for the great reply. I'm sure you're right about lugging the engines. It doesn't sound like they are lugging unless I try to use one at a time though.

You are a bit off on one bit of info though. Chevy 350s are cheaper and easier to repair and get parts for than the old and hard to find OMC Stringers.

 

[hwsiii]

Re: Repropping my cabin cruiser.

OK, I understand.


H

 

[rs2k]

Re: Repropping my cabin cruiser.

A reman stringer runs around $4000 while you can get rebuilt marine 350s for $1200.

Can I assume that going from 14x21 to a 14x19 might take the engine RPMs from 3850 to at least 4200? Can I assume that a 14x17 might make the engines get up to 4600?


I put in the following numbers to a prop slip calculator:

http://www.rbbi.com/folders/prop/propcalc.htm
3850 RPM.
1.5 gear ratio
21 inch prop
44.5 MPH

I received 0.12816326530612243 slip

Does this sound right?

 

[rs2k]

Re: Repropping my cabin cruiser.

Sorry, I had the gear ratio wrong. I either have 1.49 gear ratio or 1.41 gear ratio. I do not know which one.

If I have 1.41 than the slip is 0.1804734693877551
If I have 1.49 than the slip is 0.13397551020408183

 

[hwsiii]

Re: Repropping my cabin cruiser.

Rs2k, we keep getting information in bits and pieces, I know you don't intend on it being this way but it is. I will ask you to fill out a form that will have all the information I need to do a full program analysis of your boat and motor. With this information I believe I will be able to help you decide on the correct pitch props you need, but also please let us know the exact make and model of the props you have now and any work that has been done to them since they were bought.
Working over the internet and not being able to see the boat or prop the question is about, and most people don?t know the make and model of the prop, makes it much harder to propose a solution to the correct prop that you need. When we get information in bits and pieces we are continually having 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. 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 and RPM. ALL of my calculations are done assuming an aluminum 3 bladed prop will be used, when changing to a SS 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
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 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 or trim tabs
12. Make, model, diameter, pitch and whether SS or aluminum prop
13. WOT RPM and speed from prop, how much gas and how many people in boat for test data
14. 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

 

[rs2k]

Re: Repropping my cabin cruiser.

1. 1980 IMP Kansa
2. 28' x 8.5' weight ? 6000 lbs
3. 3 - 4 people. 100 - 25 gallons of fuel. 100 lbs of equipment... (Hopefully 200 lbs of fish )
4. fishing (Trolling and running to fishing spots) A lot of trolling will take place just below planing speed.
5. The hull is pretty darn close to a 45 degree angle but I cannot accurately measure this. Please see images below.
6. Twin standard rotation 260HP 1980 Chevy 350 powering a pair of 1980 OMC 800 Stringers
7. 260HP each. Either 1.41 or 1.49 Gear ratio. 80% sure it is 1.41
8. Per service manual. 4200 RPM - 4600 RPM
9. See image, anti-vent plates are roughly even with boat bottom when engine tilt is full up. This equates to 7 - 8 inches above the keel
10. N/A
11. No / No
12. OMC SST 387520, 14x21 stainless steel prop, both are standard rotation, cupped blades.
13. 2 people, 1 big dog, 40 gallons of gasoline, 100 lbs of equipment, calm day, no wind, 3850 RPM at 44.5 MPH.
14. Can't plane or remain on plane with one engine. Full throttle RPM is too low.

 

[45Auto]

Re: Repropping my cabin cruiser.

I wouldn't be surprised if you can't get that thing to plane on one engine no matter what you do. I've driven a 2006 26' Sea Ray Sundancer (cabin cruiser) of about the same weight with single 300HP 5.7 MPI. It took everything it had to get the boat to plane. I found it very unimpressive.

With a single 260 and single outdrive (no other engine or outdrive in the boat) I think your boat would be marginal to plane. Trying to plane it off one engine with the drag of the other dead lower in the water is going to be tough.

Good luck, I'll be watching to see how it works out for you.

 

[rs2k]

Re: Repropping my cabin cruiser.

Thanks for the info. This boat did have the option for one 260 HP when new and being a stringer drive I can lift the outdrive almost completely out of the water. When the boat is trying to get out of the hole and when on plane the raised stringer is completely out of the water. If you discount the asymmetrical thrust then the only thing I have to deal with is the extra 800 lbs cause by the extra engine. I guess it would be like trying to plane a single engine Kansa with 4 extra men. If it makes a difference, I can plane the boat with one engine at around 1500 RPM and the other at full throttle.

I'm actually a bit surprised with the performance of this boat. I knew someone with a twin fuel injected 305 powered 29' searay that was a foot wider and could barely plane at all. It's top speed was less than 30 MPH. I was expecting similar performance with this boat. This boat doesn't have any problems planing, zigging, or zagging with both engines on.

 

[hwsiii]

Re: Repropping my cabin cruiser.

Rs2k, if both of your motors are producing full power I show you need 18" pitch props. But some of my numbers indicate they may not be producing a full 260 HP each, can you check this by checking all the barrels are opening at wide open speed, compression is good and timing and advance are working right, plugs etc. Also after doing this bring it to full speed on both engines like you already did then back off on one engine and then do the same with the other engine.



H

 

[rs2k]

Re: Repropping my cabin cruiser.

Can the lack of full power just be from lugging the engines with a prop that's too large?

I know for sure that the carbs are opening all the way. I also know that the timing is fully advanced at 3850 RPM and identical on both distributors.

I suspect that the port engine is using more fuel than the starboard engine though. I recently had some trouble with fuel filters clogging up do to "junk in the tank". The port engine was always the first one to loose RPM when the filter started to clog. Both engines are fed by a single tank outlet and single electric fuel pump that Y's off to the engines after the fuel filter. Fuel flow has no reason to be different.

I will go ahead and check compression on all 16 cylinders.

As far as the ignition goes, the last owner put in 2 HEI distributors, 16 high performance wires, and 16 iridium spark plugs. A bit excessive for a fishing boat in my option.

 

[rs2k]

Re: Repropping my cabin cruiser.

I guess it doesn't have much to do with anything, but after about 10 hours on both engines there has been no oil usage and the oil is still gold. I don't think there is any mechanical problem with the engines. I will of course verify this and get back to you though.


Thanks for the help!

 

[junior1113]

Re: Repropping my cabin cruiser.

agree with hwsii 18" pitch. with 2 motors you gain a little more rpm than with one when changing prop sizes. 17 pitch 4 blades might do the trick to get 1 motor to plane boat.

 

[rs2k]

Re: Repropping my cabin cruiser.

So assuming that both engines have good compression should I go with 14x18, 14.25x18, or 14.5x18?

 

[junior1113]

Re: Repropping my cabin cruiser.

i would go 14.5 biger the track the more low end power the smaller diameter props are more agressive in most cases which made for speed. why not 4 blades? great torque. can run on plane at lower speeds great manuverability. id put 4's on there that thing would plane easily

 

[hwsiii]

Re: Repropping my cabin cruiser.

That will depend on exactly which props they are, not just diameter and pitch, tell us whih make and models of props you are looking at. Has any work been done by a prop shop on the SST's you are running now by a prop shop.


H