Prop tuning - any comments/suggestions?

[GiMLit]

Short story. I bought this boat as a project and a project it has certainly been. Floor, stringers, stripping other boats and selling parts to gather what I wanted for this one including 3 other power plants until I got what I really wanted and other than a complete exterior cosmetic redo this coming winter I am where I hoped to be with just a bit more fine tuning. Learned tons and have had a blast both working on it and playing with it with family.

INFO /Specs
1977 Cobia 18VBR i/o Bowrider
Weight - no real idea
L 18' 6"
w 6' 6"
Dead rise as I understand it it would be considered a deep deadrise with chines
1983 3.8 l (229 cubic inch) Chev V6 with 450 holley 4 bbl Holley Aluminum Intake Engine in good condition but not new, tuned 100% - OMC originally rated for 185 HP with original 2bbl -
Rated 4200 - 4600 RPM
OMC 800 series mechanical shift 1.77 ratio
Prop 15x17 OMC 391200* cupped Al prop (rated in OMC manual for a 20'-26' cruiser good for 34-45 MPH) prop in very good condition no nicks etc... KNOCK ON WOOD!!!!! I bought this one to replace one that was redone and was mint until it was a tangled MESS!

No foils, fin or tabs
Antivent plate 3/4 inch above keel


My test data
All speeds in MPH as per GPS and RPM per Fluke 88 Shop tach.

Top Speed - trimmed to get top speed - 42.5 MPH @5225 RPM
Top Speed - trimmed max down 39.1 @ 4845 RPM
Both read with - 1 person 2 - 18 gal fuel tanks full 1 anchor assorted jackets and safety gear.
Top Speed - 38.4MPH @ 4880 RPM - 8 people 2 tubes, 1 wakeboard, 1 cooler and same fuel and safety gear and jackets.
Planes @ approx 14 mph @ 2250 RPM loses plane approx 21 MPH @ 2760 RPM

Throttle response at 34 MPH @ 4200 or any other is great, no real complaints other than excessive top RPM and not tickled with the ability to plane and stay on plane, I always want more and I am pretty certain I won't stop trying anytime soon.
Usually have 6-8 people and tube and wakeboard as well as cruise.
Suggestions or comments of any kind are welcome and appreciated.

Thanks
GiMLit

 

[hwsiii]

Re: Prop tuning - any comments/suggestions?

GiM, in my opinion your motor should be turning about 4,800 RPM with the upgrade. You are currently running a 17" prop with cup which theoretically acts like an 18" pitch prop in speed and RPM. You are currently showing a 16% prop slip even with 1 person in the boat and that is very high for that boat and motor combo. You need to change to a 19" Solas Amita 4 blade prop and that should bring your RPM down to about 4,700 RPM and lower your prop slip. Here are my numbers showing my calculations, if you will notice I changed the pitch to 18" because of the cup in the prop and the 4th blade will probably take between 200 and 300 RPM off the RPM as well.


Prop Slip

Prop Change

H

 

[GiMLit]

Re: Prop tuning - any comments/suggestions?

H I really appreciate your input.
I have been wanting to post for sometime now but I tried numerous engine height levels as well as adjusting the amount of trim before posting. I achieved the best TS and overall performance and I figured it was now prop time.
I am like so many others on a budget but still performance is a big part of my entertainment so here we are.
I was SHOCKED when I went out this year with the 4bbl on and had only minimal speed increase. I took the damn fin off (I was just trying it since it was already mounted on the drive I had acquired late last season) and was sooooooo happy instant TS increase. Then I played yet again with engine/drive height and also total trim levels and got to what I was fairly certain was best over all performance.
Now I will try this prop and again play to make certain I am at optimum adjustment. Just to clarify this is the unit you are suggesting
http://boatpropellers.iboats.com/OM.../6636/?**********=921440946&*******=621302535

I will be buying from iboats not only to support this forum but due to the fact that I am 100% impressed with each purchase I have made through them.
Are there any specific results you would like to see when I test it? I thoroughly enjoy the test portion of all this. Having raced circle track for many years testing is the only way to see if what works on paper is correct or not. Paper lies to often!

I will be ordering as soon as I have your confirmation on the part #.



Thanks
GiMLit

 

[hwsiii]

Re: Prop tuning - any comments/suggestions?

GiM, I agree with you wholeheartedly that Iboats is excellent and they are definitely doing a service for the boating community with this forum. And hydrofoils are definitely not the panacea everybody thinks they are, and in faster boats they can actually slow them down and be dangerous at times.
A long time ago I had a 23' Thunderbird trihull with a 165 Mercruiser 6 cylinder and I changed the motor for a larger block with more horsepower and a 4 barrel. The *** thing was slower than it was before I changed it, it had the wrong cam in it. LOL So I know how you feel, I sold it to my brother.

Yes, that is the prop I am recommending for you.
Please get back with me and give me your WOT RPM and speed for my database when you get the new prop.


H

 

[GiMLit]

Re: Prop tuning - any comments/suggestions?

Have to eat dinner, grab my cc from the truck and the prop will be ordered. Hopefully they have it in stock and can get it to me by friday (ah maybe not) and if so Saturday weather permitting I will have results for you.
DAMN lol Aug 13 which I know will be sooner but not this week :-(

 

[hwsiii]

Re: Prop tuning - any comments/suggestions?

Hopefully they have it in stock.


H

 

[GiMLit]

Re: Prop tuning - any comments/suggestions?

Well Aug 13 is the current arrival date of the prop to my house. Now knowing iBoats has been early each time I have had something shipped I expect it sooner. Now the fact I have Aug 14th off work means I will testing my prop by then at the latest, barring serious weather issues of course.

 

[sho3boater]

Re: Prop tuning - any comments/suggestions?

You might have to change the cam and free up the exhaust to realize that 4bbl carb. Maybe port the heads a little not sure. Typical for a well sorted engine to perform the best, I mean matching components. The carb will help top end rpm like it is, but now you are beyond the cam a little, etc.

 

[GiMLit]

Re: Prop tuning - any comments/suggestions?

You might have to change the cam and free up the exhaust to realize that 4bbl carb. Maybe port the heads a little not sure. Typical for a well sorted engine to perform the best, I mean matching components. The carb will help top end rpm like it is, but now you are beyond the cam a little, etc.

Yes I agree.
In fact I happen to, not by accident, have a cam matched to this intake carb set up purchased in the transaction (if you want to call buying a brand new never touched carb, intake, camshaft lifters and springs purchased in 1986 for $780 for $50 a transaction along with spare heads and exhaust manifolds sitting ready to be played with.
My attention this fall and winter will be focused on redoing my hull but if time and cash permit I will be doing motor playing as well. If not I will likely do it through the season next year.
Love playing and seeing results, expected or unexpected.

 

[Dhadley]

Re: Prop tuning - any comments/suggestions?

Then I played yet again with engine/drive height and also total trim levels and got to what I was fairly certain was best over all performance.

How are you changing the engine/drive height (not trim level)?

 

[GiMLit]

Re: Prop tuning - any comments/suggestions?

A bonus of OMC Stringer drive >>>>
I can change height by equally raising front and rear stringer heights (engine mount height).
I can change total trim levels by unequally changing stringer heights (engine mount height).

 

[hwsiii]

Re: Prop tuning - any comments/suggestions?

That is a unique idea, an excellent thought pattern GiM.



H

 

[GiMLit]

Re: Prop tuning - any comments/suggestions?

TY sir.
I am a little on the cheap side and anal as well, quite a combination really. As well I love to see the results of testing different things, the racing back ground helps you exhaust most any option, as well as think outside the box. I am far from finished. As soon as I get the prop it will be tested in current trim and then trimmed in a couple different ways to see what if any positive results changes have. This may not be the ideal prop but I will find it's ideal settings with my set up and then try to make it better again.
By my calculations we have about $1000 into this package now (incl prop on it's way).
Boat, galvanized trailer (acquired last year for negative $500 plus a bunch of very good spares) spares for most everything as well as tubes, wakeboards, jackets etc....
Such fun!
Think I have settled on 2 possible names for the boat now.
'Fun For US'
OR
Distraction!

 

[GiMLit]

Re: Prop tuning - any comments/suggestions?

Well prop arrived today
I am busy all week until Friday but I don't think I can wait that long to see how it tests out.
Maybe I have to squeeze in a run to the lake after work tomorrow for a half hr of testing!

 

[hwsiii]

Re: Prop tuning - any comments/suggestions?

I will be ready to run siome numbers.


H

 

[GiMLit]

Re: Prop tuning - any comments/suggestions?

I will be ready to run siome numbers.


H

ARE YOU READY????????????????????
Before the numbers I am pleased with the performance of the prop all around but I have lost a bit of hole shot (yes I knew it was going to occur)

46 mph @ 4760
oh so nice sweet spot purrs like a kitten and sounds like it could do it for days on end 41 mph @ 4350

trimmed all the way down 39 mph @4350

added a 1/2 inch to front mounts to reduce total trim up and add to down
TS 44 blips into 45mph @4650

All of these are with only me and my gear aboard and tanks full of fuel.
h what do you think ???

I have a 14.5 x 19 al 391201*cupped that needs a bit of fixing up, I know of a shop that fixed a prop for me previously and did a good job.
What do expect the results would be ? prop tuning now to get what works the best and then.............
I still have the cam to try and I expect abit more grunt from it but can't back it up as I don't have specs to back it up. Once I nail this all down with data recorded I expect to change out the cam and then maybe play with a little cam timing to maximize performance. Funny I am doing this with a 1977 OMC. Many /most people would think I am daft. Maybe I am??? lol but I sure am having fun!

 

[GiMLit]

Re: Prop tuning - any comments/suggestions?

dang hwsiii I thought you would be all over this one LOL.

 

[hwsiii]

Re: Prop tuning - any comments/suggestions?

You were able to drop the prop slip where it should be and the RPM are just where I think they ought to be as well.

Prop Slip

Ask them to put a double cup in the 391201 and it ought to work real well.


H

 

[GiMLit]

Re: Prop tuning - any comments/suggestions?

have to save up a few dollars first, with any luck I can do it before our holidays at the end of August.
Thanks very much for your help.

 

[hwsiii]

Re: Prop tuning - any comments/suggestions?

You are very welcome Gim.


H