Zip transom height w/ short shaft

Outboard designs up to 14'

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RLASON52
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Zip transom height w/ short shaft

Post by RLASON52 »

Am I right in assuming the "15" measurement" when talking about a short shaft engine, refers to the distance from the top of the transom "relief" to the bottom of the keel? That distance appears to be the default on the Zip plans Thanks Rick

gdcarpenter
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Re: Zip transom height w/ short shaft

Post by gdcarpenter »

You are correct in your assumption. If you already have your engine you can measure up from the anti cavitation plate to the underside of the engine mounting brackets. The "Rule of Thumb" is that the anti cavitation plate should be level with, or below the bottom of the keel, to keep the prop submerged.

Running your engine higher may help top speed, with less of the outboard's foot in the water, but too high could lead to cavitation in hard turns or starvation of water to the engine.

Though many prefer the 'retro' look of the short shaft (15") they are somewhat more difficult to come by. May consider building transom full height if you have not yet selected an outboard engine, with provisions for cutting it down later if need be.
This is my first, last and only boat build.

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vupilot
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Re: Zip transom height w/ short shaft

Post by vupilot »

Be aware that not all short shafts motors are actually 15". My Mercury from 1975 is actually 16.5". Looking back on my build there isnt much of anything I would do differently but I think building with a 20" transom is the one thing I would change if I could. Short shafts are getting too difficult to come by used with remote steering. Most of what I find is tiller steering. It can cost about $500 to convert tiller to remote steering.

You can still order engines brand new with short shafts and remote steering though what I am finding out is engines over 25hp the dealer usually is required by the mfg to install the motor and not allow you to just pick up the crate and take it home loose. [quote][/quote]

-Chris

John K
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Re: Zip transom height w/ short shaft

Post by John K »

I found a short shaft that also was 17 inches. I installed a lift so I adjust the lift to make sure the motor was not too far into the water.

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mrintense
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Re: Zip transom height w/ short shaft

Post by mrintense »

You should also verify if the measurement is made vertically rather than along the transom. By this I mean that if the transom is mounted at an angle, then the measurement should be vertical rather than along the transom surface.

I am not positive this is the way it is on the Zip, but on my Vera Cruise I had to measure straight down from the keel for the cut out rather than laying the measuring tape on the transom skin. Hope this makes sense.
Carl

Crafting a classically styled Vera Cruise

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chipper363
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Re: Zip transom height w/ short shaft

Post by chipper363 »

Many of the old Merc short shaft are 16.5 The distance on ths tramson is 16.86 from top to bottom for a engine height of 16.5

Chipper

dmac
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Re: Zip transom height w/ short shaft

Post by dmac »

Does one measure from the top of the transom to the bottom of the keel or to the bottom of the boat?

gdcarpenter
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Re: Zip transom height w/ short shaft

Post by gdcarpenter »

Middle of the transom, measure down from the level top of the transom that the engine will sit on down to the bottom edge of the hull.

At a 12 degree angle measuring vertically or along the angles back of the transom will likely net a whopping difference of about 1/4"! Depends how fussy you

Most older engines clamp to the transom and have a line of holes lower down for through bolting. You might consider through bolting one up from the bottom in case you ever want to change the mounting height.
This is my first, last and only boat build.

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galamb
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Re: Zip transom height w/ short shaft

Post by galamb »

Here's a little diagram that shows some of the terms.

To get a "near perfect" mount you want to line up the bottom of the cavitation plate with the bottom of the transom (lower edge) at the keel line (whether the keel is totally within the hull or extends below).

In this case this is one of the benefit's of building yourself. Get the specs of the motor you plan to use and then make your transom cutout so that the motors cav plate will line up perfectly.

Mounting the motor "lower" will create a little extra drag which can cause it to burn more gas and increase your "draft" but otherwise there is no mechanical dis-advantage (so you could put a long shaft motor on a short shaft transom without raising it, but you would have 5 extra inches hanging below the boat so would have to be careful in shallows).

Mounting the motor a little "higher" can increase your top end speed a bit, reduce a bit of drag and save some gas (but generally talking about 10ths of a gallon in the best conditions). This however, requires a lot of piddling around to get it correct. If you go too high you get cavitation which results in "loss of traction" and you could suck up enough air to damage the impeller in your water pump.

Anyhow, here's the pic -
Attachments
Shaft Length.jpg
Shaft Length.jpg (16 KiB) Viewed 4423 times
Graham

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Andy Garrett
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Re: Zip transom height w/ short shaft

Post by Andy Garrett »

On my Zip, I had my motor hanging on a donor boat. I took careful measurements from the rearward bend in the bracket opening where the top of the transom is seated. Specifically, my measurement was from the aft edge of the transoms upper surface width.

I used a log shaft motor and it worked out perfectly. A yardstick flush with the keel is also flush with the cav plate of motor. Just allow yourself some area to 'trim' the top to fit if you need to. It always better to have and not need than need and not have. :wink:
Andy Garrett

Perhaps the slowest Zip build in Glen-L history...

gdcarpenter
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Re: Zip transom height w/ short shaft

Post by gdcarpenter »

Happened to take my ZIP out for a run today to try refurbished prop. It's powered by a 1959 Merc Mark35A, considered to be a 15" short shaft. As you can hopefully see in the photo it's actually 16 1/2" from top of transom engine mount horizontal surface to bottom of 'Vee' on hull. That's why it's best to have a known engine on hand before committing to cutting your transom.
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This is my first, last and only boat build.

http://www.gdzipbuild.blogspot.com

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