(Prop Help??) 115 Mercury on a 14' Cobra Cat Tunnel Hull

[rjcamel2355]

Hey Guys, I recently acquired a 14' tunnel hull boat, it has a 4cyl mercury 115 on it, with a 13x21P Quicksilver prop...

I could be very wrong, but I think this needs a lot more prop, the boat (I was told) is a kevlar hull and only weighs about 200-300lbs).

It is jackplatted, and the previous owner said he had ran a cleaver style prop on it previously.... My question is what would you guys recommend? I know there are a limited selection of props that are 13", and there is no way I could fit anything bigger than that, due to the gearcase. I personally believe it needs a 28+ seeings how my 17' skeeter with a 200 is turning a 26P 4 blade at 6000...

Hints tips suggestions?

 

[flyingscott]

What RPMs are you running now? That motors rpm range is 4750-5250 rpm so you need to prop for there.

 

[Texasmark]

That engine runs 4750-5250. Propping for the top end as I always did and had the best performance, would put your rpms at 5250. Gear ratio is 2:1.....engine makes 2 revs per prop rev. Assuming with that boat and a high mounted engine, AV plate above slipstream by 1" and correct trim, would estimate your slip at 10%. Going to the go-fast.com BAM prop slip calculator (easier than doing the math each time) and using the 13 +/- range props that fit your lower unit, 24P Ballistic XL sold by iboats, gives you 60 if a screw in wood (no slip) or considering the 10% realistic slip of 54 mph. If your slip dropped to 5% then you should be at 57.

I ran that prop on my last rig, cell phone shot of the wake (what little there is of it) is my Avatar. Engine was 90 hp and 2.33 gear box. My engine was rated at 5500 and I ran 56-5800 area with it for 48-50 gps.

You wanted a number, there's a place to start. The wheel on your 200 is for the larger gearbox and is the 14 +/- inch diameter grouping. Don't know if you can find a cleaver for your 4 ? gear box but don't know if you don't ask/look.

I did buy a 14+ prop (from iboats) for my 90 and it fit. It was a Turning Point Hustler and I think it was 14 ? and 21P. I had about ? clearance between the prop blade tip and the bottom of my AV plate. I was running the thin Merc trim tab already and had notched the front of it to accommodate my Ballistic so there was no need to tweak that for the Hustler prop. If you go with the 14, I wouldn't go larger than 1 ⅜ which would be cutting it close. If you want to go fast, a smaller diameter and higher pitch is the solution......large diameters go with shallow pitches for pushing heavy loads.

I do know that Merc/Mariner offered several 150 hp engines and some had the 4 ? gearbox, others the 4 ?, back in the 1990's (13" and 14" dia. ranges). They offered 3 gear ratio's: 2:1, 1.87:1 and 1.78:1. So high pitched props to fit your gearbox are out there and iboats is a good place to start looking.

 

[rjcamel2355]

Definitely just looking to make it go fast, I guess I was expecting a lot more out of the setup than what realistically is there.

 

[Chris1956]

gee, I would think that set-up would do 70MPH or more. Can you try that 26P prop on her?

 

[Dukedog]

cobra or kober?.. true tunnel?... pictures?...shaft length?... needs lwp and motor jacked up (pressure gauge a must) if not already... run any kinda prop ya want as long as it is "over tha hub cleaver"...gonna need ta do some huntin' for one/several.. not ya everyday prop... doesn't need anything that produces any bow lift at all.. that will hurt ya!.. power trim (if ya have it) can/will sometimes be your enemy...

 

[Watermann]

To get the speed from your set up you would need to go back to a clever type solid prop rather than a through the hub exhaust style. When you price them out you'll see why the guy didn't sell the prop with the boat.

 

[Dukedog]

join tha Scream and Fly board.. there are several over there that run tha "short tunnels".. put ya post up on General Boating or Props, Setups, Riggin'.. probably finda prop there also... would still like ta see a picture or two.. i ran a 15' kober kat with a 225 merc many years ago ('80 ta 'bout '84).. a little exciting and very, very quick!.. was set up for tha 1/4..... PICTURES PLEASE?

 

[rjcamel2355]

I found this prop. It is the removable hub style. Says 23p signature on it. Can I run this style prop?

http://imgur.com/8EPTvwr
http://imgur.com/oaKuzcg
http://imgur.com/dlADWKa

 

[rjcamel2355]

And here is the boat Dukedog

http://imgur.com/0m6Dzs4
http://imgur.com/Si8GlrO

 

[Dukedog]

"kober kat".... lake/crusin' version.. little heavier rigged out like it is.. mine was all out race version and touch under 300 lbs, BARE hull.. no nothin'....

prop ya show is right blade design... tha thru hub "may" be ok with tha weight its carrin'.. pitch is gonna be determined by height ya can run and rpm and you wanna turn tha little motor... motor height you can run will tell tha tale!.. wish i had a little more experience with tha little motors on it but i don't... everything 'bout it will be different from anything i ran on mine... one thing for sure is ya need a water pressure gauge so ya can get tha height right.... with stock gear case its gonna limit this aspect of it... any weight ya can remove will improve it.. tunnels like that are very weight sensitive...

in tha "i wanna go fast" world ya gotta have $$$ ta spend on props, they make or brake tha whole set up thing....... most everything else is adjust, this take this out, move this up or down etc.............

jan... be very careful with that thing.. doesn't take much, if any trim......

 

[rjcamel2355]

One battery two seats and a 6 gallon tank aside from the little stuff a hotfoot and the control box.

Water pressure gauge is already ran and ready to go. Right now the cavitation plate is even with the tunnel. I'm sure it needs to go up some. I've only got to run it with the stock 13x21p quicksilver prop that's on it.

 

[rjcamel2355]

Already relocated the battery to the front of the boat in the center.

 

[Dukedog]

that may not work out so well with side steer anda little motor.. do things as a "test" first.. batt on port side, rear with tha side steer.... start a fund for a V6 and either "dual cable" (need even with little motor on it) or hyd. steering... move ta center steer with tandem seating.... most times "wanna go fast" turns inta "gotta go faster".. little steps and be safe as in kill switch and quality PFD!.. it can blow over, even under 70!.............

jan... forget everything about your skeeter... nothin' 'bout it applies here!

 

[Dukedog]

forgot ta ask.. any prop shops in your area?.. there are many 13 spline yamaha cleavers out there and believe they will work on your merc with tha right hardware.......

tha one pictured is a ron hill... 50/50 on how it will run if its close on right pitch.......

 

[rjcamel2355]

There is one nearby. But.... I've not heard very good things about them...

Ill definitely shop around if you have something you recommend using though

 

[Watermann]

Your 115 Merc is an inline 6 TOP right? You didn't say what year the motor is but 90 through 150 HP Mercs were I6's. They have a huge following and were made well into the 1980's. There's a very active facebook page for the Inline 6 with guys who love these motors. https://www.facebook.com/groups/213725501742/

 

[rjcamel2355]

Just a 4 cyl with 4 carbs. Oc189295 is the serial number on it

 

[Dukedog]

shoulda ask first thing.. is all tha wood ie: transom, floor etc. still solid?

not gonna be much help with what tha little motor needs.. like said not very familiar with 'em.. one thing though.. how many holes for water intake on each side?

 

[rjcamel2355]

Ill have to get out and count them. The floor transom and transom were completely re done new carpet and paint also. Its all solid from what I can tell