converting 34ft twin inboards to outboards

True.. but the least sensible way of trimming a planing boat is to add stuff - effectively ballast. Better to move the stuff you already have. Better still, pull stuff out altogether making the boat lighter

 

I considered measuring the hull cross section at various stations in order to calculate bouancy and then to try to put the boat on four stands and level it -then weigh it at each point- if I can figure out a relatively easy way to do this. I can rig up something with a small scale and a long lever... then I would have some real numbers. I guess I could compare that to what the boat originally had for weight and CG- am I thinking on the right track?

 

I wouldn't go to that extent - at least not for now. Just do the weights and moments calc that I described. If the 2 numbers are vastly different, then you can worry about doing more serious investigations.
In fact you can soon set up a spreadsheet to estimate the effects of moving other things - like batteries - still without having any more info

 

I should calculate the moments about the bow - shouldn't I use some point astern of the bow?

 

you can use any point on the boat that's easy to measure from. Designers will generally use the intersection of the bow and the designed water line (station 0) but this is just for convenience - you could just as easily use a point 20 ft forward of the bow if you wanted to! I thought the bow would be an easy place for you to measure from, that's all

 

Thanks for the help. I'll do some measuring and do the calcs.

 

CG moment calculations

Williamson,
I did the measuring and moment calculations. The waterline at the bow is 4.6 ft. behind the most forward point of the bow (at the deck - I'll call this the bow tip). I calculated moments from the waterline at the bow. I also calculated moments from my swag of where the CG of the boat is without any engines or running gear (19 ft. from the bow tip, or 56% of the way back from the bow tip - the boat has a lot of cabin and flybridge forward of this estimated CG point). I tried to be conservative with my approximations - guessing the inboard parts a little light and a little forward. I didn’t put the weight of the bracket/swim platform into the outboard calculations. I’m removing a wooden swim platform. The bracket/swim platform will be designed with flotation, so it should more than offset the weight. I calculated moments for the V-drives, prop shafts, struts and rudders.

I used 950# each for inboard engines with transmissions – this is what the current 5.7L Crusaders weigh – and some of these boats have big blocks in them.

The outboards I am planning to use are 250hp 30” Evinrude ETECs, Evinrude says they weigh 530#, I used 550# for props, fuel, steering stuff.

Here are my numbers:

Moments from the bow waterline:

Inboards = 56k lb-ft.
Outboards = 35.7k lb-ft.

Moments from the swag CG:

Inboards = 22k lb-ft.
Outboards = 20k lb-ft.

It sure looks to me like I won’t have any CG problems with this installation! I do have a lot of room belowdecks to move fuel, water, batteries, etc. if I need to adjust CG.

What do you think?

 

OooPS! I meant Willallison....

 

I'm a little confused by your numbers....
working backwards from your results, I calculate that the CG of the inboards is 29ft aft of the bow and the outboards are 32ft aft of the bow. Surely this can't be right...? - or are the inboards mounted right at the transom?
Even so, by the time you put the o/b's on a bracket (which I gather you haven't included in your calcs..?) I would expect the distance between the two to be greater...

All that aside, if we take your numbers as correct, then you will have moved the boats CG further fwd - so she will trim down somehat by the bow at rest. You will probably have to move some other things aft in order to correct this.

 

better numbers

Will,

Ohhh....I found a math error- Here are the corrected calculations:

34C 1978 Silverton inboards vs outboards wt and balance
bow at 0+00 arm calc
waterline at bow 0+4.6 ft. 4.6
transom at bottom of hull 0+33.3 ft. 19
outboards at 0+36.5 ft.
my guess where the CG is of the boat without engines 0+19

moments about the waterline at the bow:
INBOARDS
wt, lbs. station arm moment
engines 1900 24 19.4 36860
vdrives 300 26.2 21.6 6480
prop shafts 80 28.5 23.9 1912
struts 30 28 23.4 702
rudders 60 32 27.4 1644
total 2370 47598

OUTBOARDS
engines 1100 37 32.4 35640

moments about station 0+19 (my guess for CG of empty boat)
INBOARDS
wt, lbs. station arm moment
engines 1900 24 5 9500
vdrives 300 26.2 7.2 2160
prop shafts 80 28.5 9.5 760
struts 30 28 9 270
rudders 60 32 13 780
total 2370 13470

OUTBOARDS
engines 1100 37 18 19800

What do you think about these numbers? My guess where the CG is is just a quick eyeball guess... I have these numbers in a spreadsheet so I can change the arm easily...

I stuck 0+14 in for the CG of the empty boat, and the moments for both engines are about equal at 25,300 lb-ft.

Any additional help is appreciated!

 

Will,
I hope you can read the numbers - I copied them into this forum's message space then added spaces to make the columns all straight, but it doesn't look the same - having problems with typing now ok, here are the moments about the waterline at the bow, which is 4.6 ft. behind the bow:
inboards 47,600 lb-ft
outboard 35,600 lb-ft

I also calculated about a point 19 ft. behind the bow, which I made an uneducated guess that that's where the CG of the boat without engines or drive gear is. Here's the moments for the engines about this point:
inboards 13,500
outboards 19,800

If I calculate the moments about a point at 0+14, both engine/drivetrain moments are about equal at 25,300 lb-ft.

 

I dont like sums, probably because i was never any good at it.

I think your wasting your time, you cant work out all this with numbers.

Bolt the motors on and put it in some water, take it from there.

1 miscalculation and its all a waste of time.

I appreciate that on cold winter nights it is fun to play but I wouldnt rely on them in the slightest.

Sorry but theres only one way to do this.

 

Jack - what exactly do you think designers spend all those years studying for? Of course there's a point to it. True, there are those (very few) old time designers who conjured up vessels without doing any maths at all. But remember that for every designer who was succesful at doing it that way, there were (and still are) a thousand who's boats turned out to be complete dogs. You think boats just pop out of the mold and float level on their lines by coincidence...?.... nope - it's as a result of calculations - and lots of 'em!

Your numbers (and I haven't checked them) still indicate to me that the boat will float bow down somewhat. Take a look at some of the other things on board that you might be able to move - batteries for instance.
With your spreadsheet (assuming that you've written it correctly!) you should be able to plug a few ot these other things and see how easy it's going to be to bring the boat back into trim.

I'm not suggesting that you go moving these things based soleley on these clacs - at best they'll ony be an approximation - but they should give you some idea of whether you can trim the boat correctly without having to add ballast... which on a planing hull is definitely the enemy!

BTW - I'm not sure how you come up with the numbers being the same at this +14 measurement. If the moments are different from one point, they should be different from all others. The point you measure from doesn't really matter - just do it from wherever is convenient.

 

I didnt say there wasnt a point to it. I said your wasting your time. This is not a new boat out of the mould. I dont think any designer would be brave enough to draw an exact water line on an unknown unlaunched boat.The designer of my new boat certainly was'nt . I stood back and had to guess myself to antifoul before launch,-- you know i was very close. No help from him at all.



The quotation above Will, is basically what I said. I agree its just an approximation--at best. Thers only one way to know.

 

On the inboard installation, I calculated the moments of all the parts - the engines with transmissions, the vdrives, the prop shafts, struts and rudders - all seperately then added the moments together. This is the same as lumping it all together into 2370 lbs. at 20.1 ft. from the bow waterline for a moment of 47,600 lb-ft.

The outboards are 1100 lbs. at 32.4 ft. from the bow waterline for a moment of 35,600 lb-ft.

If the moments are calculated about the point 20.1 ft from the bow waterline, the inboards moment is 0. The outboards moment would be 13,500 lb-ft.