Outfitting Klamath 18 CC with 2017 60HP CT trying to figure where to start on Prop size. Probably aluminum.

Saltydog#1

Recruit
Joined
Aug 24, 2023
Messages
4
Outfitting Klamath 18 CC with 2017 60HP CT trying to figure where to start on Prop size. Probably aluminum.
 

Saltydog#1

Recruit
Joined
Aug 24, 2023
Messages
4
The best info I can find is somewhere about a 13.5 X 16. The Klamath is a light boat 1200 lbs.
 

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,666
Do a www search for go-fast.com prop slip calculator. The gear ratio is the raw number not the X:1 whole thing. If it's 2:1 insert a 2. Then plug in your prop pitch in inches....like 16 you mentioned. Then plug in your engine's recommended max RPM like 5500. Then solve for max speed and then guess at your slip, with what you said about your boat I'd guess at 15 (%). Then solve for the actual speed that this combination may provide for you.

In looking for your answer you would do best to be in relatively calm water and trim the engine for best speed which is very important as if the bow is too low (at speed) there will be more hull in the water than necessary causing excessive drag and reducing your top speed.

The best thing is to have a prop already so that you can get max speed (which will include prop slip percentage since you will have the "actual" max speed), not the calculated max speed number with zero prop slip, and the engine RPM number at that speed to plug into the calculator. Once you have that number, you can obtain the prop slip % by adding .

Depending on the prop design you select, and the type of boating you desire, there are stern lifting type props (most aluminum props fit this category....listed as general purpose, that do well on heavy, underpowered boats, deep Vs and such. Then there are bow lifting props (usually more expensive Stainless Steel types, made for light, high speed boats.....as I said they get the bow up and reduce hull wetting and get the best speed.

You might get the 16 aluminum as you said, hundred bucks or so and get the baseline you need to calculate the right prop then keep the 16 as a spare.

My thoughts. Hope this helps.
 

QBhoy

Fleet Admiral
Joined
Mar 10, 2016
Messages
8,342
The best info I can find is somewhere about a 13.5 X 16. The Klamath is a light boat 1200 lbs.
Remember it’s a CT gearbox. If it’s anything like the other CT 2.1l mercs (like my 115 version), you’ll need the larger (and better range) type props, for the larger gearbox size.
I run a 19” or 20” 3 blade tempest plus or enertia on my 19ft cuddy and 115 ct. 43-45 mph at 6100/6000 rpm roughly
 

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,666
Remember it’s a CT gearbox. If it’s anything like the other CT 2.1l mercs (like my 115 version), you’ll need the larger (and better range) type props, for the larger gearbox size.
I run a 19” or 20” 3 blade tempest plus or enertia on my 19ft cuddy and 115 ct. 43-45 mph at 6100/6000 rpm roughly
Isn't the CT gear box a casting used on the higher HP 75 thru 125 HP Mercs running a 2.33 gear ratio? I have 2 of those and they run a 13+ diameter prop. whereas the 60 with regular gearbox would run a 10+ inch prop?????
 

QBhoy

Fleet Admiral
Joined
Mar 10, 2016
Messages
8,342
Isn't the CT gear box a casting used on the higher HP 75 thru 125 HP Mercs running a 2.33 gear ratio? I have 2 of those and they run a 13+ diameter prop. whereas the 60 with regular gearbox would run a 10+ inch prop?????
It is. Up to 115hp these days.
Just thought that a 13x16 would be an odd size for it.
There is certainly a 16x16 that would fit it
 
Last edited:

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,666
It is. Up to 115hp these days.
Just thought that a 13x16 would be an odd size for it.
There is certainly a 16x16 that would fit it
I ran a 14" dia. Hustler on my 90 Merc for a short period just for grins. I don't remember where I obtained it, not the pitch. Obviously I didn't like it as it didn't stay on but a few minutes and back to the dock to put on something else. Had 1/4" clearance under the AV plate. Had the short trim tab and the pitch wasn't all that much so I cleared that.

I assume that you mean 16" props are probably available in 16P?????

I assume Salty dog had shopped around found a 13x16 and that was why he mentioned it. Thinking about how props increase in diameter as you reduce the pitch, I would think that a 16 if it existed, would be more like 13 3/4" diameter.

Merry Christmas
 

QBhoy

Fleet Admiral
Joined
Mar 10, 2016
Messages
8,342
I ran a 14" dia. Hustler on my 90 Merc for a short period just for grins. I don't remember where I obtained it, not the pitch. Obviously I didn't like it as it didn't stay on but a few minutes and back to the dock to put on something else. Had 1/4" clearance under the AV plate. Had the short trim tab and the pitch wasn't all that much so I cleared that.

I assume that you mean 16" props are probably available in 16P?????

I assume Salty dog had shopped around found a 13x16 and that was why he mentioned it. Thinking about how props increase in diameter as you reduce the pitch, I would think that a 16 if it existed, would be more like 13 3/4" diameter.

Merry Christmas
Merry Xmas to you too.
You know, and maybe I know…but not exclusively and for sure…you’re dead right in the thinking that I also have…
As prop pitch increases…traditional thinking might suggest that diameter reduces a little.
So I was just surprised to read of a 13.anything by 16” pitch, in terms of the 4.3/4” gear case. I’m used to like a 14x19” then a 13.75x21” maybe. 13 something less again x23”.
Anyway, generally an odd size of pitch might be the norm for that size of gearcase.
But I’m also aware that there are many odd and even and anything in between sizes these days. An enertia 20” prop I have now plays testament to that.
Perhaps I was just warning of the mistake many can make, around thinking the CT 75-115 of around 2015 to current day, can be compared in size to the previous Bigfoot size 60-100/115 or previous years. They didn’t adhere to the current rule and sizing requirements.
Certainly with my 115ct and as far as I know, the 75-115ct…they follow the requirements and sizing that the 150+ gearcase has…the very same even, that the mercruiser alpha one and bravo one has too.
Additionally, and at the risk of boring an audience, in my opinion and many others, the CT is and has proven, the best way to go.
Despite the original recommendation around the CT being exclusively for heavy boats, I absolutely think this may not be totally accurate, for the most part.
Should you be lucky enough to have the CT gearbox, it means that you can have the option of running a much larger and wider range of props. A range that far outweighs the thinking or idea that a smaller and standard gearcase, with any ideas of less drag (but along with a much less developed or standard of props available) meaning a better performance outcome, might boast.
The only circumstance that a non CT engine might be a better option in any way…I can only imagine, might be a very specialised bass boat with a 115 pro xs…airborne and aired out, with somehow, a prop that’s available off the shelf that allowed such a thing.
But 99 times out of 100…the CT and its propeller range available…has got to be a better option.
Finally…and to back up the thinking above.
I’ll leave you with the performance figures and data from my own 115CT powered boat. And please bear in mind that I’m not a 4 stroke fan or anything close, historically.
19.5ft fletcher 19gto cuddy cabin
1000kgs dry
Full camper hood, fuel and family onboard
19” tempest plus 3 blade, 43mph gps @6000-6100 rpm
20” enertia 3 blade, 45/46mph gps @5950-6000 rpm.
The above are easily and all day, any day achieved figures.
Despite not being anything ridiculous or equivalent to the other two fast boats I have or most any usual boat over where you guys are…it should surely be considered impressive, given what she is. 19-20ft cuddy, camper hood and plenty onboard…only a badged 115hp…and a sure thing 43-46mph gps !! Not bad, I think.
There is a 3.0 mercruiser inboard version of this boat, more common. They will never see a genuine 40mph gps. Well set up examples will do well to see 36-38mph with their 140hp, running light and no camper up.
There is a 4.3 carb too. They can boogie to be fair, seeing mid to late 40’s gps with 190-220hp.
But again…back to the 115 CT on mine. Achieving almost what the 4.3 can…fair play to her eh ?
Finally…there is a version of this boat that was rated to and fitted with, the 135hp mercury black max v6 2 stroke. Legend of an engine for sure….
Hate to say it (being a 2 stroke man to the bone), but this identical boat I have with a 115CT merc 4 stroke…will absolutely keep inch for inch pace alongside. She had even seen off an identical hull’d open 19ft version (bowrider) with a 125hp 2+2 merc. Wasn’t even close.
Enough from me. But you get the point.
Wee video hopefully attached, showing fairly accurate speed, fed from nmea gps and her coming on and off the limiter at 6100, then down to 5900…building back to limiter and so forth. Pretty sure she sees around 45/46 gps here.
 

Attachments

  • IMG_1055.mov
    47 MB · Views: 0
Last edited:

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,666
I guess I was somewhat narrow minded on thinking that the CT would only be the next size up gearbox....never thought they might up it a notch and make some engines with the 150+ gear case which would allow for the 14+ diameter wheels .

Your numbers are as you say, quite impressive. Nice chat!
 

QBhoy

Fleet Admiral
Joined
Mar 10, 2016
Messages
8,342
I guess I was somewhat narrow minded on thinking that the CT would only be the next size up gearbox....never thought they might up it a notch and make some engines with the 150+ gear case which would allow for the 14+ diameter wheels .

Your numbers are as you say, quite impressive. Nice chat!
Cheers Texas. Merry Xmas
 

Saltydog#1

Recruit
Joined
Aug 24, 2023
Messages
4
Thanks everyone. I bought a 13.25 X 17 we shall see. The Klamath is very light weight. I'll do my best to keep ya'll posted.
 

Saltydog#1

Recruit
Joined
Aug 24, 2023
Messages
4
Hi, so the 13.25 x 17 seems good. We have to get four men in the boat to get full approval. It might be a little too top endish. But the CT should bring the lower end into play ok, once loaded. When we get a few more runs in I'll update. Thanks to all who inputted.
 

Attachments

  • Klamath CT.jpg
    Klamath CT.jpg
    2.1 MB · Views: 1
Top