Short Shaft Motor or Long Shaft Motor for Zip, which one??

Outboard designs up to 14'

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Thomasbuilt
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Short Shaft Motor or Long Shaft Motor for Zip, which one??

Post by Thomasbuilt »

I was wondering which motor is better for the Zip??

I'm sure old vintage engines that are long shafted are more common.

Just would like to know which preforms better, may a one helps the boat planes out quick with one or the other, I'm not sure just throwing some thoughts out, any help??

thanks for any info.

upspirate

Re: Short Shaft Motor or Long Shaft Motor for Zip, which one??

Post by upspirate »

Vintage motors are normally short shaft 15 3/4" inches in the 30-40 HP range

Later models are normally 20 3/4 and are easier to find,parts etc,they have electronic ignition,different ratio oil mix etc,better for salt water corrosion resistance.

Either will work fine as long as the transom is the proper height for the motor used.

They changed the way they rate the HP...can't remember when they did this, so a newer 40 may not put out the same as an older one.

All this is comparing motors of similar condition

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vupilot
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Re: Short Shaft Motor or Long Shaft Motor for Zip, which one??

Post by vupilot »

The power rating was changed in the early 1980's I think 83. The power is now rated at the prop shaft not the crankshaft. So a newer 35hp should push a Zip along about as well as a pre 1983 40hp, all other variables being equal.

As for short or long shaft, its completely personal preference. Just build your transom to suit which you decide on. The transom height should equal the motor shaft length. 15" shaft=15" transom. 20" shaft=20" transom.

Having built a short shaft Zip I think its slightly easier to hook up steering and other controls on a long shaft because the motor is up higher and the steering (especially the tilt tube type) is up higher and out of the way of the motorwell and you will not have any clearance issues to deal with from the steering rod and link.
In the picture below you can see I had to cut out a clearance port for the steering link in the motorwell side. To get around this with a short shaft you can use a motorwell mounted steering bracket or make the motor well wider.
Launch 003.jpg
If you look at the link below you can see on Tom Drake's Zip with a long shaft the steering is all above the deck and motorwell. A easier installation in my eyes (T. Broadlick Photo)
http://picasaweb.google.com/Buckeyesmit ... 7786680802

Again its all personal preference and each work equally as well performance wise. Long shaft motors do seem a bit more common but I really wanted the low sitting motor and the petite look of a short shaft.

-Chris

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Re: Short Shaft Motor or Long Shaft Motor for Zip, which one??

Post by John K »

vupilot wrote:The power rating was changed in the early 1980's I think 83. The power is now rated at the prop shaft not the crankshaft. So a newer 35hp should push a Zip along about as well as a pre 1983 40hp, all other variables being equal.
If I remember right the older Mercs were always rated at the prop and Johnsons/ Evinrudes were rated at the crank. That is why Mercs would also outrun the others.

I don't know where you could find this, but that is what I remember.

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galamb
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Re: Short Shaft Motor or Long Shaft Motor for Zip, which one??

Post by galamb »

So many versions of when - so I'll add my 2 cents.

It was the early/mid 80's when outboards began to see some Shaft Horsepower (SHP) ratings instead of Brake Horsepower (BHP).

Merc completed the total transition in 1986 when they launched their entire new line of outboards. To keep in step with them OMC completely followed suit - so by 1986, at least all North American produced outboards (Merc, Force, Johnson/Evinrude) were rated in SHP.

Some were new designs, others were simply "rebadged". The general rule of thumb with outboards is that the SHP will be about 10'ish% less than the BHP rating. (It can be calculated but there is no "one equasion fits all" rule)

So in a couple notable cases, the 40 Merc became the 35 and the 200 Johnson became the 185 (an odd horsepower that OMC scrapped a couple years later). So while the 1983 200 Johnson and the 1984 185 Johnson were mechanically identical (ditto the 40 and 35 Mercs), the new "stickers" simply reflected the rating change.

As a side note, I don't know of anyone that could "feel" the performance difference between a 35 or 40 horse motor on a given boat. If you took off the decals and told them it was a 50 hangin' on the back, they would take your word for it. In the 30-50 horse range moving up or down 10 horses results in "no noticable" difference. Above 200 horses even a 50 horse change would barely make a difference, so when comparing a 40 BHP compared to a 40 SHP the only thing that should concern you is 1) can I get parts and 2) which will look "cooler" on my build :)

Until the EPA got into the mix and started really regulating outboard emissions, Merc tended to "under-rate" the horsepower of their motors so that they would always come out on top of "head to head" tests. As an example, most 90 Merc's built between 1985-1994 actually would measure at 100 SHP if they were dyno tested.

One note on transom height. The coastguard rates a boat with a 20" transom "safer" than one with a 15" transom. That will allow you to carry more weight or hang a higher horsepower motor on the 20-incher and still be in compliance with the guidelines.
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Thomasbuilt
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Re: Short Shaft Motor or Long Shaft Motor for Zip, which one??

Post by Thomasbuilt »

Thank you guys for setting me straight on the motor info, like always I learn alot more than I thought I should know.

Even thought the plans call for 40hp max, I've seen pics of 45's and one 50hp on the Zip in customer photo section, but I notice the plans have a insert which may be something they send for all outboard motor boat plans stating how thinck the transome should be all the way up to a 85hp. I'm sure that is just common boat building methods. There is no way you can put anything bigger than a 50hp on a 200-300lb hull?

Have a good one and thanks to all you guys for the help.

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galamb
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Re: Short Shaft Motor or Long Shaft Motor for Zip, which one??

Post by galamb »

The Coastguard has different formulae for calculating the maximum safe horsepower for a given hull and it is based on dimensions, not the hull weight.

Most "homebuilt" outboard powerboats will be based on a hard chine, v-bottom design.

So you can use their basic formulae to calculate what they figure is "safe" (and if you don't meet their regs it's near impossible to insure the boat and many States/Provinces won't register it).

You will need a couple measurements to figure max horsepower. First you need the boat length - measured from the upper tip of the bow straight back to the exact middle of the transom (often referred to as "length overall" or LOA) - this is usually a different number than either the waterline length or centerline length.

The second number you need is the transom width. And that has to be measured along the upper edge (top) of the transom - in that case it doesn't matter if the transom flares in or out at the top (most hard chine boat chines are their widest at the top) - whatever the measurement is accross the top is what they use.

If you build your Zip exactly to spec it would be 14' 4" long and have a beam of 5' 9" (and while I don't have the plans to look at, have to assume that the transom is 4-6" narrower than the widest part of the beam - so for the sake of doing the calculation call it 5' 4").

Now here's where you get the calculator. The first thing you calculate is the "factor" which is simply the length X transom width -

(these are coastguard numbers not mine :))

(14' 4") 14.333' X (5'4") 5.333' = 76.437

Round your factor to the nearest whole number - 76.437 = 76 (76 is your factor).

Now you use one of two formulae depending on whether you have a 15" or 20" transom.

If you have a 15" transom -

Multiply your factor by .8 = 76 X .8 = 60.8
then subtract 25 = 60.8 - 25 = 35.8
then raise it to the nearest multiple of 5 = 35.8 raised to 40

So for a 15" transom with the measurements of the actual hull, the max horsepower would be 40.

If you have a 20" transom -

multiply your factor by 2 = 76 x 2 = 152
subtract 90 = 152 - 90 = 62
then raise it to the nearest multiple of 5 = 62 raised to 65

So for a 20" transom, all other measurements the same, the max horsepower would be 65.

(these formulae only apply to V-bottomed boats with factors above 52 that have either a 15 or 20 inch transom)
Graham

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A "professional" is someone who gets paid for their work - it doesn't necessarily mean they are good at it :)

Tom Drake
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Re: Short Shaft Motor or Long Shaft Motor for Zip, which one??

Post by Tom Drake »

You ask for the time of day and you are told how to build a clock. Your question was answered by Warren and Chris in the first two post. It's a matter of choice. As an RC flyer, which do you prefer--JR or Futaba ? Same type of question. But, the side trip was interesting. In the late 40's, we owned a Mercury Hurricane 10. The street story was that it equated to most 15HP outboards because Mercury rated their outboards behind the gearbox at the prop shaft while others rated infront of the gearbox. That was a simple answer that worked at the time. 10HP was the max for a racing hydro class at the time and Merc dominated that class.

Thomasbuilt
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Re: Short Shaft Motor or Long Shaft Motor for Zip, which one??

Post by Thomasbuilt »

LOL, your right. I know how to build a clock and a motor now, :-). Thats what I love about this forum. You guys answer every question and make me think of more questions I should of been asking myself. Thanks again for all the additional info, it is alot of help. Have a good week.

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Re: Short Shaft Motor or Long Shaft Motor for Zip, which one??

Post by Lachlan1 »

Sorry to open up a (very) old Thread but I’m wondering if anyone would know of a formula that would calculate the extra force a long shaft motor would put on the transom and what kind of reinforcements would be required. I’m looking at a new Yamaha 40hp outboard for my zip build and it comes in short or long shaft versions. As I understand, under acceleration, the motor would essentially be pulling the top of the transom back/aft. Seeing that with a 20” transom it is higher than the deck, would it require extra supports and how substantial would they have to be?

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galamb
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Re: Short Shaft Motor or Long Shaft Motor for Zip, which one??

Post by galamb »

I don't have a calculation but can give you the (theory) that with maybe a little time on google would give you such a formula for calculation.

What you are looking at is the difference in "leverage" between the "shorter" short shaft and the "longer" long shaft and how much more stress/pressure it would place on the point where it is fixed to the transom.

Think of it like a teeter-totter. The further away you apply a (force) the greater effect it has because of "leverage".

Same with the motor. If the prop is further away, even though it is still only producing the same amount of thrust, because of the leverage it is putting greater stress on the top of the transom.

In the case of a 40 horse motor (modern with HP measured at the prop), at the prop you are producing about 1080 pounds of thrust (maximum).

Horsepower and thrust don't exactly correlate to each other (because thrust is a measured value and horsepower is calculated by moving "X" weight a specific distance in a specific amount of time) but "close enough for comparison purposes", 1 hp is roughly equal to 27 pounds of thrust - this is why Electric Trolling motors are 27 lbs, 54 pounds (plus or minus a pound) so that they "roughly equal" a 1 hp or 2 hp gasser.

Sorry, I digress, so the calculation you need to find is "how much" would 1080 pounds of thrust effect a stationary (point) applied to a lever at a distance of 15" compared to 20". If the answser was "linear" you would guess 25% "more" with the longer shaft, but I suspect it is probably logarythmic which would see it (imparting) twice or three times the force due to the leverage.
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Bill Edmundson
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Re: Short Shaft Motor or Long Shaft Motor for Zip, which one??

Post by Bill Edmundson »

It is a linear relation. Force times distance.

Bill
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vupilot
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Re: Short Shaft Motor or Long Shaft Motor for Zip, which one??

Post by vupilot »

You can easily add an extra 1/2" ply to the transom to resist any added flex of the long shaft. You can also add corner blocks to all the frame work that ties into the top of the transom and motor well. With those additions the forces on the water would not concern me at all. Is your boat already built?

What does concern me with larger heavier motors of today compared to 1954 when the Zip was designed is the forces of the heavier motors when towing down the roadway hitting potholes, bumps, etc. A transom saver helps while towing but going down the highway always made me a lot more nervous about the transom. Never had a worry on the water. Go with the lightest motor/lowest HP your can get away with is my advice.

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Re: Short Shaft Motor or Long Shaft Motor for Zip, which one??

Post by Hercdrvr »

A new 40 hp Yamaha is 215 lbs, Sounds like a good engine for the Zip if your transom is built to specs with quality materials. I’m with Chris on road chop, I cringe watching the Malahini in my rear view mirror over bumps, but it’s been bouncing around back there for 3 years now with no major damage.
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Re: Short Shaft Motor or Long Shaft Motor for Zip, which one??

Post by hoodman »

I'd say if you can get one a short shaft will look way better on a zip than a long shaft. It's such a low sleek hull. They are great little boats.
Matt

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