Author Topic: Should I get a 25 or 30 inch shaft motor?  (Read 8167 times)

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xrayengineer

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Should I get a 25 or 30 inch shaft motor?
« on: October 20, 2012, 08:08:46 PM »
I am building a 28 foot GA.  Mine will have a 30 inch outboard bracket with swim platform.  Should i get a 25 or 30 inch shaft motor?  I may buy a motor next week at the Ft Lauderdale boat show.

Here is my bracket

Brian.Dixon

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Re: Should I get a 25 or 30 inch shaft motor?
« Reply #1 on: October 20, 2012, 08:38:13 PM »

You might want to talk to a local dealer that puts on brackets like that.  The wake rises behind the boat but I can't say by how much at 30" behind the boat.  For sure I wouldn't go with a longer shaft motor, but I don't know how the top of your motor mount bracket aligns with the cut-out that I designed for a 25" shaft either... I think you'll have to take some measurements, draw it up, and go talk to someone that installs brackets like that and get their opinion.  And with the motor so far behind the boat and only a little buoyancy in that bracket (probably zero when on plane), you risk having the CG too far aft and possible porpoising... have you planned ahead for how to counter the weight of the motor being that far back?  Say, by moving the heaviest items in the boat forward (tanks, batteries, anything else heavy)?  Don't want to see you disappointed or having to add ballast in the bow...

Brian


xrayengineer

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Re: Should I get a 25 or 30 inch shaft motor?
« Reply #2 on: October 21, 2012, 07:34:18 AM »
Hi brian,
I did run this bracket idea past you a few months ago.  You sent me a drawing for the full transom with no notch and indicated that you would bolt the bracket on and it should be fine.

I was going to calibrate the boat by moving the single fuel tank forward and back when the boat is complete, minus installing the rear deck and mounting the fuel tank.  I was going to take the boat to the water and move the fuel tank fore / aft to get the CG correct .

I would appreciate your thoughts on this.  Do you remember my questions about the deck height, scupper height and that I would not allow the deck to be close enough to the waterline to allow water to come back in the scuppers?

I can mount this bracket for either a 25 or 30 shaft length.  There is plenty of room either way.  The motor is 3-1/2" higher with this bracket than if mounted direct on the transom. 

A 30" shaft will allow the powerhead to be 5" higher out of the water than a 30".  But it will also raise the CG slightly.  A 30" shaft motor will get less saltwater dunking while drifting in seas, that is the only difference in this application.

The bracket is a 30" setback.

When you have a chance, can you run a calc and tell me the amount of weight difference and inches of CG shift this will create?  The motor is going to be a 250hp 4 stroke from Suzuki or Honda and weigh around 575pounds.

I am painting the boat this week and flipping right after the paint cures!

Thank you,
Jim Stancil

Brian.Dixon

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Re: Should I get a 25 or 30 inch shaft motor?
« Reply #3 on: October 21, 2012, 10:19:04 AM »
OK, that answers a few questions for me (thx).  Did you tell me that the bracket moved the motor 30" behind the boat?  For some reason, I was thinking it was closer to half that.  Do you already have the 250hp outboard?  Adrian P. in Oregon has a 250 and took his boat to about 47, but still had more throttle leftover at that speed.  His waterline is the same as a 28' boat but it's because it's a 26 footer with sponsons on either side that were made by extending the hull.  I don't remember how far back his bracket put the motor, but I'm thinking it's about 2/3rds of yours at most and he picks up some porpoising above 47 ...a hint that the CG is too far aft.  You may need to put batteries forward as well... but first, I do need to estimate your CG and get back to you.  Too bad dropping to 200 hp doesn't save much, if any, weight.  You might get better fuel efficiency though, since I'm convince 250 hp is too much ...you might not be able to use it all. 

I'll get back to you...

Brian


xrayengineer

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Re: Should I get a 25 or 30 inch shaft motor?
« Reply #4 on: October 21, 2012, 12:33:38 PM »
Hi Brian,
I was only looking at getting the 250 because the smaller HP motors are usually made using the same motor but just de-tuning it.  You end up with the same weight motor.  However, if you are convinced that the boat could not ever use that much power, then I can save several thousand dollars and get a smaller HP motor. 

I have no idea what the final weight will be.  Here are a few items the boat will have that will add some weight...16000 BTU marine A/C unit located under one of the v-berths in the front...stereo system with 6 speakers, 32 gallon livewell...the entire pilothouse and cabin will be made with 1/2" MDO board....standard electronics, radar and auto pilot...marine head....two batteries...Yamaha Generator (75 pounds) ..90-100 gallons fuel....30 gallons freshwater...standard fishing tackle, safety gear

the bracket weighs around 120#, and has around 240# of positive flotation in the tub(net gain of around 120# of flotation).

Like I said, I am going to get the boat nearly 100% ready, including getting the motor installed and rigged, and leave the rear deck off.  Then take the boat down to the boat ramp and float it.  Then I will slide the fuel tank and move the water tank in order to hopefully get the COG correct. 

In your FAQ, you make this statement:

Waterline at the stern, typical displacement (3800-4000 lbs total): 4 inches (add about an inch for each additional 650 lbs or so)

I don't quite understand what that means.  Does that mean that the Deck Height at the stern, which should be 1/2" higher than the stringers, is 4 inches above the waterline?  If that is true, then it seems that when I go to "calibrate" the boat I should try to get the waterline back to that point?  I am assuming that with the 30" bracket lever arm and a 575lb motor that the waterline would drop around 2" at the transom.  Adjusting the COG would be better than raising the deck height, I assume.

Thanks for the help with the COG!

Jimmy

Brian.Dixon

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Re: Should I get a 25 or 30 inch shaft motor?
« Reply #5 on: October 21, 2012, 05:06:26 PM »
I'm glad you're taking a systematic approach to balancing the boat.  Looks like you do have some weight options, e.g. you can put the freshwater tank in the crash chamber if you have to etc.  And no, I do not think you need more than around 200 hp or so, 225 hp max ...which you probably would never use unless you load the boat up to close to 6000# and still want to go fast.  There are no strength issues and generally, the ocean will keep you from getting to go too fast, so paying for extra horsepower isn't worth it.  You need enough for a good hole shot and hard fast maneuvering at slow speeds (turning into scary big waves for example), but for your 28 footer, 200 and up will be plenty for all these conditions.

For that waterline statement, imagine the boat floating level in the water at the dock.  For the 26 footer, loaded to a 4000# displacement, the waterline will be about 4" above the chine flat.  You can measure it by putting a ruler or stick flat across the chine flat and then measure up from there.  This is a conservative waterline and you should not feel obligated to do anything to match it.  "Conservative" means the boat is more stable if slightly deeper in the water, e.g. because it increases the waterline length a bit.  Now, the statement about submersing another inch for each 650# or so goes, that assumes the additional weight is distributed such that the CG stays the same, e.g. boat still floating level as you load it up.  Adding weight behind the CG (or center of buoyancy) will cause the stern to trim downwards (and bow upwards).  There are 2 issues associated with a CG that is too far aft.  For one, lowering the stern makes it just that much easier to allow waves into the boat, and once one big one gets in, it becomes even more susceptible to a wave coming in ...this is the most common capsize scenario.  Ideally, for a 28 foot boat, you'd want the bow to trim up about 2" or so, so a certain amount of trimming low at the stern is acceptable.  The second issue associated with an aft CG is stability.  As the CG moves aft, the boat becomes more susceptible to porpoising.  Porpoising may come and go at different speeds, exist in some water conditions but not in others.  Believe it or not, smooth water is generally the worst ...just when you want to go fast!  A small chop seems to be the best for stabilizing the boat.  Anyway, boats float ...they are like a teeter-totter balancing on their CG.  More forgiving than airplanes to load, but not as forgiving as trucks and cars!  Regardless, we'll try to take a close look at your boat's CG and do whatever we can to keep it all balanced correctly.  With a boat this size, there are so many options in what a bloke can do, it's not a bad idea for all builders to run their ideas through the ringer to see what comes out...

Brian


xrayengineer

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Re: Should I get a 25 or 30 inch shaft motor?
« Reply #6 on: November 04, 2012, 06:07:30 PM »
I did some mockups with the bracket...it will need a 25" shaft.  The cool thing is, the setback of the bracket of 30" will allow the motor to be 3 1/2" higher than if mounted at the transom

Brian.Dixon

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Re: Should I get a 25 or 30 inch shaft motor?
« Reply #7 on: November 05, 2012, 03:11:54 PM »
I did some mockups with the bracket...it will need a 25" shaft.  The cool thing is, the setback of the bracket of 30" will allow the motor to be 3 1/2" higher than if mounted at the transom

Many of the outboard manufacturers publish data that says "Raise the motor an inch for each 12" of bracket" ...but how high your wake rises behind the boat really depends on a) how much the boat weighs (displacement) and b) the hull form.  From my experience, the wake rises higher than they predict.  I think they're being conservative.  Anyway, go-fast boat owners will sometimes raise the motor until water pressure in the cooling circuit starts to drop off, then they lower the motor just enough to get the pressure back up.  Theoretically, this is as high as you can go and is also supposed to be the most efficient height ...noting that on a planing hull, the CG is always higher than the bottom of the boat and the prop's thrust is always below the CG.  If it were possible, you'd get the best efficiency when the thrust is in alignment with the CG.  This is why brackets can sometimes make a boat faster or burn less fuel.  Taking advantage of that rising wake allows you to have your line of thrust closer to the boat's CG (vertical CG that is).

Brian


xrayengineer

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Re: Should I get a 25 or 30 inch shaft motor?
« Reply #8 on: November 05, 2012, 06:17:18 PM »
Brian, when you look at the new COG calcs for my boat bracket, i should tell you i am going to have a single aluminum fuel tank, around 80-100 gallons.  I was going to have the tank made shallow enough that it could go far forward of need be.  It is going to be around 8 feet long.

I want the boats attitude to be as you designed it to sit in the water, and the deck height at the rear to be 4" or higher above the waterline.

Brian.Dixon

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Re: Should I get a 25 or 30 inch shaft motor?
« Reply #9 on: November 05, 2012, 11:14:46 PM »
I'm still working on my new, US Navy type (and more complete as a result), CG spreadsheet... But note that having trim and inch or two off in the bow or stern on a boat this size is not a show stopper.  If the bow trims too low, it encourages bow-steering and therefore is definitely the greater of two evils.  Ideally, you'd want a 25-26 foot boat to trim about 1-1/2 inches high at the bow, and no more than about 2 to 2-1/2" high on the 28 footer.  This tends to keep the boat's bow working as it should when under way, and it also results in bilges draining aft where they can be pumped out... very important, and much better to work this way than having to add forward bilge pumps as a 'just in case' thing because the boat's trim is off.  A slight bow-up trim also helps decks self-bail out through scuppers in the stern as well, but this can be artificially induced by putting exposed decks on a slant to force them to bail aft.  OK, so on the other hand, if the boat trims too low at the stern, it may mean the CG is too far aft and risk porpoising (worst case), or being in a stern-low situation when at rest or adrift, and that makes it easier for water to come in over the stern ...and once in, the boat trims lower and can take another wave etc ...the most common scenario for capsize (too much ice, fish, coolers, gear, and fishermen in the stern...).

Brian


xrayengineer

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Re: Should I get a 25 or 30 inch shaft motor?
« Reply #10 on: December 13, 2012, 08:18:44 PM »
Any new updates on your new COG spreadsheet?

Brian.Dixon

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Re: Should I get a 25 or 30 inch shaft motor?
« Reply #11 on: December 13, 2012, 08:56:09 PM »
No ...been busy lately and in/out of town on business.  I'll probably work on it over the Christmas break.  I'm off from 12/21 through 1/2 this year.  Got 14" of snow in the last day or so ...just in time.  My daughter's coming up for Christmas and wanted a white Christmas in Alaska, but all we've had is dry brown frozen ground until this last storm came through!

Brian


Ed Snyder

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Re: Should I get a 25 or 30 inch shaft motor?
« Reply #12 on: December 30, 2012, 07:53:54 AM »
Hello Brians daughter  :D
I was to be with my daughter in NZ as I type this, but the boat GODS didnt let it happen, they expired my passport! o well.

xray I was going to get a 300 HP Honda....... purry nuts lol, now going with a 300HP diesel inboard, with a stern unit, just like having a cheaper runner, got 800 to 1800 KLM trips planned, theres the Indian Ocean off the coast here in W.A. Australia, no shelter for ever!
So need power to run for it, and diesel to make it affordable, having said that, diesel here is 10c dearer than petrol a Lt.

Brian I have more info for the C of G calc's, maybe u can send me that spreadsheet so I can try work it out?
Simpsons multipliers otherwise.... long way around it!

Have you thought aboat a design 3 ft wider?

Happy new year all  :D ;D 8)
Not waving....... Drowning!

Brian.Dixon

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Re: Should I get a 25 or 30 inch shaft motor?
« Reply #13 on: December 30, 2012, 08:22:46 AM »
Any new updates on your new COG spreadsheet?

Gents ...I am indeed making progress on the CG spreadsheet.  Hope to have it done within a couple of weeks...

Brian


Vicent59

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Re: Should I get a 25 or 30 inch shaft motor?
« Reply #14 on: September 18, 2014, 07:24:31 AM »
Can I mount a 30" shaft motor on the transom directly?  Thanks.  Vincent