High Thrust Motors on Tunnel Hulls

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snookiehunter
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High Thrust Motors on Tunnel Hulls

Post by snookiehunter »

I am trying to find some discussion on this, with little luck. I'm building a GF18 w/ tunnel, (from XF20). I plan on using a 4-6" hyd jack plate. I am at the point where I really need to pull the trigger on a motor. I found a decent deal on a Merc 60 4st Bigfoot. I have little experience with (full size) boats or motors. I've always run jons w/ small tillers. I have NO experience with tunnels, and sizing/ picking props. My thoughts on the Bigfoot are;

Lower gearing, (2.33:1) equals larger prop. This is both an advantage, and a disadvantage.
Advantage being that I will have much more prop selection, and much more bite, in the water helping with any tunnel cavitation issues.
Disadvantage because a larger prop equals a deeper draft. The Bigfoot is turning up to a 14" prop, compared to standard turning a 10". This puts me two inches lower, not including the skeg. ( I don't know how much longer it is)

I will assume that speed, is really a moot point, because lower gear ratio, and higher pitch prop, will equal higher gear ratio, with lower pitch prop. Also, because I am less concerned about top end speed, than a shorter/ quicker hole shot.

I often fish alone, but I would enjoy seeing good performance when I decide to bring along three buddies, and plenty of beer.

This puzzles me, because I'm not sure if the pros outweigh the cons. I have read quite a bit about all of our tunnel hulls having issues with bite. Am I correct in thinking that the lower gears, and 14" prop are a great advantage? I read that this lower unit uses a 4.5" hubbed prop. I tried to research what is avail., and was lost in a sea of information! :help:

TomW1
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Re: High Thrust Motors on Tunnel Hulls

Post by TomW1 »

Snookie a lot of good questions. Some people feel that using a High Thrust lower unit is a disadvantage but it really isn't. You still turn the prop at 6000 rpms you just have to pick the right pitch to go with the diameter of the prop.

Both the Mercury Bigfoot 60 and the Yamaha T60 are similar with a 2.33 lower unit and in fact Yamaha built the Mercs until the late 90's early 2000's. The specs on both are almost identical and I would not hesitate to buy either.

First lets get rid of the ventilation/cavitation definition. Most boats and props never see cavitation it is caused by water boiling off the lower unit and into the propeller, causing pitting. Ventilation on the other hand is caused by the propeller losing its grip on the water in turns and when raising the prop to high.

I'm going to leave you to the guys who have built tunnel boats on how far to set back the motor.

As far as sizing and picking a prop I can help you there. We got Cracker Larry's new build for Rogerdog right on the first try. It always depends on an accurate weight, motor choice, waterline and a few other options you put on the boat. We will probably be looking at a 13 1/4 x 13" prop. But let's not speculate now. Let's wait until you finish the boat and you are about to launch.

You are right instead of a 10 or 11" diameter prop you will use a 13 to 14" diameter prop, which pushes the skeg down an extra 3" plus or minus. I would not worry about this two much as it is the same as the F70 you have been looking at and the reason for the tunnel.

Good luck and I think you would not be disappointed with that motor and the right propeller.

Tom
Restored Mirror Dinghy, Bought OD18 built by CL, Westlawn School of Yacht Design courses. LT US Navy 1970-1978

snookiehunter
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Re: High Thrust Motors on Tunnel Hulls

Post by snookiehunter »

Am I correct in thinking that there are more prop options available for the larger gear box? 13"-14" range? I recall Jorge saying that there just wasn't enough avail for his F70.

Also, thoughts anyone, on proper set back for this arrangement? I'm thinking to go with a 6", In order to raise the motor a little bit higher. I hope I can afford the weight aft, going with the 260 lbs Merc.

Again, this is the XF20 tunnel, built to plans. I have no experience with any tunnel hull, so I value your experience greatly. Especially XF 20 builders.

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Re: High Thrust Motors on Tunnel Hulls

Post by Cracker Larry »

I don't think you will have any problem with the weight. Rogerdog's F90 weighs 370 pounds and we've got a jack plate with 5" setback. It handles it fine.

Qualifier, this boat is not a tunnel, I don't know squat about tunnels, or how much that might change the floatation.
Completed GF12 X 2, GF16, OD18, FS18, GF5, GF18, CL6
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Re: High Thrust Motors on Tunnel Hulls

Post by Prarie Dog »

Before I built my GF18 I looked at a tunnel hull boat built by Marine Service in Port Lavaca. I believe they are out of business at this time. They built a boat that looked like a fiberglass version of a GF18 that had a tunnel on it. Many of those boats were rigged by the factory with 90HP Tohatsu's and they had almost the exact performance numbers as my standard GF18 with a 50HP Tohatsu, except of course, the ability to get up in real shallow water. There is an old guy down that way named Jack Forman that probably knows what prop and pitch were used on those boats. If I was rigging your boat I probably would want to use the lightest 90hp available that was a known reliable motor. Oh, and don't forget a jack plate.

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Re: High Thrust Motors on Tunnel Hulls

Post by cali123 »

I built a basic XF20 and used a Bobs 6 inch setback jackplate. It is powered by an old but good 70 Yamaha 2-stroke. The water from the tunnel drops off before contacting the motor so I would recommend as little setback as you can get.I use a Powertech 13"x12' triple cup prop and it grips well.

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Re: High Thrust Motors on Tunnel Hulls

Post by TomW1 »

Yes there are more propellers available for the larger gear box. Most will be 13" or 14" in diameter with pitches running from about 9 - 20 " or more. Rake and cup will also be more variable. I ran the Mercury Prop Selector and it kicked backed 10 props using an estimated weight for your boat and a 2008 Bigfoot motor. Didn't know what year the used one you are looking at. So props are out there for it.

Tom
Last edited by TomW1 on Tue Jun 02, 2015 4:51 pm, edited 2 times in total.
Restored Mirror Dinghy, Bought OD18 built by CL, Westlawn School of Yacht Design courses. LT US Navy 1970-1978

snookiehunter
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Re: High Thrust Motors on Tunnel Hulls

Post by snookiehunter »

cali123 wrote: The water from the tunnel drops off before contacting the motor so I would recommend as little setback as you can get.
Thank you, Cali123. This is what I was looking for. I remember seeing a video on an XF20, that showed how the tunnel performed.Im not sure if it was Jaques tunnel, or not. How has some of the other builder's tunnels performed? Major Gator, Aripeka Angler? I'm asking so I might have a good idea where the sweet spot might be. But again, mine may wind up being different still.

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Re: High Thrust Motors on Tunnel Hulls

Post by cali123 »

The video that you viewed was likely of my boat . I built the tunnel per Jacques plans. It performs well but other types might be a little more efficient. I only make short runs so it doesn't matter. With a 13x13 triple cup prop I get 28 mph at WOT with the tunnel vent open. I'm running a 70 Yammi 2stroke. Motor height is a non issue because i'm running a Bobs low water pickup.

snookiehunter
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Re: High Thrust Motors on Tunnel Hulls

Post by snookiehunter »

cali123 wrote:I built a basic XF20 and used a Bobs 6 inch setback jackplate. It is powered by an old but good 70 Yamaha 2-stroke. The water from the tunnel drops off before contacting the motor so I would recommend as little setback as you can get.I use a Powertech 13"x12' triple cup prop and it grips well.


Thanks for the video, Cali. I remember seeing that one a while back. Thanks for the comment also, it helps a lot, but raises now more questions.
The Xf20 study plans show the tunnel making more of a wave, than what Cali's video shows. I think this is likely due to his light/ weight trimmed forward design. While it looks like it performs well, and he recommends as little set back as possible, I wonder how other xf tunnels, with more weight aft are performing. Some of the other center console, heavier versions. I also wonder how the same tunnel will perform differently, all together in a Gf18. Will the draft differences between the two hulls have any effect? I would like to pick the right jack plate, the first time. My lack of experience here, is obvious. Thanks for any input.

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