converting 34ft twin inboards to outboards

[naturewaterboy]

Anyone have any experience with or suggestions for converting a 34ft+ inboard powered boat to outboards? I have a 34ft Silverton (12.5ft beam) that was powered by twin inboards. I would like to power it with two 150hp or smaller outboards. My one concern is that I need to design a bracket for the outboards, and I believe I need enough bouyancy into the bracket to insure that when the boat pitches bow up, the outboards do not submerge. Also, I do not intend to power this boat with enough hp to plane - I'd like to be able to calculate how fast it will move with various engine sizes. I would like to get a cruise speed of 10 kts.

[Willallison]

1st question ... why?

[naturewaterboy]

Will,
I need to get thru shallow water where I'd like to raise the engines. I also like the ability to get all the metal out of the water, and easier maintenance.

I live on the water and I want to pull the boat out of the water when hurricanes come our way, and this will be easier to do at home without running gear hanging down.

I bought a boat that the previous owner had tried to swap the gas engines for diesels. It has V drives that didn't fit, shafts and struts that are shot, rudders broken, prop shaft seals bad, etc. - to put inboards back in all of this would have to be replaced, so this would be a good time to switch to the outboards.

I don't have to travel far to fish or dive, so I don't need a fast boat, and I don't want to spend a lot of money on engines. The new 4 stroke outboards also have less stinky exhaust.

[Willallison]

Ok - well a couple of points.
Planing hulls tend to make poor displacement boats, particularly in rough weather. But, so long as you don't try to push it up towards it's displacement hull speed - probably about 7 knots in your case - then it will be ok in calmish water.
For that you will not need anything like a pair of 150hp engines. Without working it out, I would think that a single 40hp - preferably one of the 'big foot' varieties - would do the trick.
The biggest problem you will find is that because you are so restricted in the diameter of the prop, you will suffer from a lack of 'grip' on the water. Coming alongside a pier will require hitting reverse several boatlengths out etc. But that just comes down to practice.

Of more importance is weight distribution. You will be pulling around a ton out of the boat and replacing it with ony a couple of hundred kg - at the most. As a result, the boat will sit higher in the water - possibly lifting the chines clear. This will make it tender at rest (rocky-rolly) and noisy as waves slap the underside of the chines.
It will definitely require some movement of other onboard weights and probably the addition of ballast to bring the trim back to it's correct position too.

[naturewaterboy]

Will,
Hmmm.... I remember reading about displacement hull speed, but my crainial neurons no longer can retrieve that info... Can the displacement hull speed be increased with any (relatively) easy modifications? What is a fast displacement hull speed for a displacement hull boat (just for comparison)?

[captword]

Anotherthing to take into consideration is the length of the shaft to get below the silvertons vee but the biggest prob is the boat. That transom was not designed for outboards and the stringer system needs to be beefed internally. If the transom is to thin or possible delaminated it has to be fixed and reinforced with the propper amount of cloth. I'm not saying that you cannot do the work yourself, but get several expert oppinions before you start.
captword

[Willallison]

Will,
Hmmm.... I remember reading about displacement hull speed, but my crainial neurons no longer can retrieve that info... Can the displacement hull speed be increased with any (relatively) easy modifications? What is a fast displacement hull speed for a displacement hull boat (just for comparison)?

short answer: no
displacement hull speed is equal to about 1.34 x square root of the waterline length. In a hull like yours it is likely to be somewhat less. The only way of increasing the hull speed is to make the boat longer (or a whole lot narrower). I wouldn't try to push your boat more than about 5 -6 knots

As far as beefing up the hull goes, you may need to do a bit, but the smaller o/b that I'm suggesting I wouldn't think so

[naturewaterboy]

Thanks for the help. I want a boat that I can take offshore - to the Bahamas from the Florida Keys. Sounds like this is a bad idea - trying to convert a planing hull to a displacement hull boat. I appreciate your responses.

[Frosty]

It could be that you have other options. You say you have 2 diesels that are running but the vdrive transmission combined is shot?

You could turn the engines round and fit a pair of jets. Same price roughly as a pair of new outboards.

[naturewaterboy]

I've gotten rid of the diesels - sold them. I really would like outboards as I do take boats into shallow water - I park boats in two lagoons that can get less than 3 feet depth. One is on the bay side of Key Largo - the wind blows from the east, and I have about 2 1/2 ft. of water. The ocean side of Key Largo - at hi tide, I have 3 feet. All other times less.

The Silverton 34C has about a 5 ft hi transom - it will definitely need to be reeingineered and reinforced, and I can get help with that.

Iya in Jakarta posted info on his conversion of a 29 Luhrs from 275hp diesels to 250hp outboards and it worked very well - faster on less horsepower and it corrected handling problems with the Luhrs. He says that they've done this to a lot (12+) boats in Jakarta.

I was thinking of having one of the bracket manufacturers build a custom hull extension/bracket for the boat. I've talked about this with one of them (forgot which one - have to look at my notes). They were proposing to extend the hull 30" - this would give some floatation near the engines that would keep them dry(er). But they have not built a bracket for a boat this size.

[Frosty]

I think that this might be a good time to look up the fuel consumtion of a 250 Hp outboard running at full. Please be seated while reading.

[Willallison]

I recall many moons back, Bertram released versions of their 25 and 28 foot boat with OMC's Seadrive's. These were basically just outboards with their own bracket designed to bolt onto the transom. The boats were much lighter, considerably faster - and yes, more economical too. And that's way back in the days's of very thirsty carburetted 2-strokes.
They did suffer from being tender at rest as I suggested before however...
You really need to decide what sort of boat you want. Whacking on a couple of big, modern outboards will almost certainly give you a faster boat - but if you are after a good rough water, displacement boat, better to buy one that's designed to be just that.

[naturewaterboy]

Hmmm..... I want a boat mostly to fish and dive locally, don't need to get out to the reef in any hurry (10kts is fine), I'm a working guy so I don't want to spend too much for the engines, the more I spend for the boat, the less I have to spend on fuel....I'd also like to take occasional trips to neighboring islands - the Bahamas and Cuba when they open it up - these are 200-300 mile round trips across open ocean.

My desire is for an economical boat - but I like the size and layout of the 34 Silverton. It is a comfortable size for us to fish and travel on. If I can put some outboards on it and run them slow and burn less fuel, I'll be happy. I know if I put 2 x 250 hp and run them fast, I'll need to get a second job to pay the fuel bill.

[naturewaterboy]

Can a boat designed as a planing hull (Silverton 34C) be used as a semi-displacement hull boat? If I were to put less 300 hp or less on this boat (inboard or outboard) would it be a safe boat to cruise offshore? What problems should I be concerned about with this setup?

[Willallison]

In saying this, I'm assuming that your boat is indeed a planing hull....
The problem with most planing hulls is thay they are pigs at semi-displacement speeds. They tend to 'bog down', poking their noses towards the sky and generally wallowing around. It's not always the case, of course, but those that do perform well at slower speeds tend to be light and feature very low deadrise at the transom.
Maybe you could post some pics of the boat - in particular that which lies bebneath the surface..

[Bob S.]

Your best bet for hanging those engines would be a Porta Bracket as they are designed to vertically lift engines clean up out of the water without tilting (if you get the long one). They will easily handle 2 engines. I would put on a pair of 150 Suzuki 4 strokes. They have a low (2.5:1) gear ratio and swing a big blade 16" dia. prop. With about a 15" pitch your boat would likely plane nicely and the weight involved would help maintain the proper hull attitude underway and at rest. Your transom is not likely cored (and rotten) so it would be pretty easy to beef up. Sounds like a good project.

[ted655]

:eek: :eek: "don't have to travel far to fish or dive, so I don't need a fast boat, and I don't want to spend a lot of money on engines. The new 4 stroke outboards also have less stinky exhaust."
Well, you are researching things, that's good. BUT here is your oxymoron.
You will find that OBs are MORE expensive overall.
By the time you buy them, the jackplates, the trim tabs AND all the controls that make everything work.... kaching, kaching,$$$$.
Find all this & p: rice them first. You will see used diesel (what you really need) is easier, cheaper to find. Is cheaper, easier to repair and is cheaper to operate. OBs are high maintaince women!
In stall just a single engibe inboard. A Detroit D is a good choise, but any other is good.
As far as the shallow water issue, start researching surface drives. There are several, the ones from Indonesia are reasonably priced. 2 OBs won't save you money, NOT unless the're junk and you are willing to have 'issues' every time you turn around.

[Willallison]

Surface drives are generally a complete waste of time in a boat of this sort. They are (generally) optimised to run at speeds in excess of 30 knots, and don't usually come into their own until above 40. True, there are some interesting developments on slower speed surface drives just coming to the for now, but they are usually intended for much bigger craft and would make the cost of an outboard seem like a joke!
The twin 150's are similarly a case of overkill. Without knowing a great deal about the boat, I'd say that they would probably not have enough power to get it on the plane, or just barely so. Further, this is a project that is supposed to be providing a cheap to build and economical to run platform. One, or two for redundancy's sake, small outboards would seem the simplest and probably the best to me.
Though, I still have to say that a planing hull doesn't make the best displacement boat, for all the reasons I've previously outlined

[Bob S.]

I agree that it would be a lousy displacement boat, that's why I suggest enough power to make it do what it is designed to do. The weight distribution problem would also be far less given 1000 lbs of engines way out back on a bracket. Additionally, the original poster has expressed the desire to also go well offshore (Bahamas, Cuba). Doing so in that hull with a pair of 40hp outboards would be folly IMO to say nothing about a total lack of market should he ever decide to sell it. The original post indicated a willingness to purchase a pair of 150's. They are 11k USD ea plus about 6k USD to rig them on a bracket, controls, gauges, etc. If that is within the budget then I submit its the best way to get the most use out of this boat.

[Willallison]

Yes - pottering across the bay is one thing. Venturing offshore, quite another. And as you say, resale value would probably be zilcl as a displacement boat

[naturewaterboy]

Hmmm... I don't want to have a boat with no resale value, and I understand that I can't make a decent displacement boat out of it. If I were to put a pair of 150hp outboards on it, it still wouldn't plane, I don't think. The factory put 260hp (350 cu in. displacement)gas inboards in the boat - and some people replaced them with 300 hp (454 cu in displacement) gas inboards and said it made a great difference - much better boat. So I'm assuming that 2 x 260hp inboard power just gets the boat on plane.

So maybe I should think about putting inboard diesels in the boat. I'd need V drives - I have two - not a pair - Walter Manufacturing - but they are the wrong style for the boat. So I need to find engines, transmissions, Vdrives, put new shafts and struts in. I can do all this work myself, but I'd really like to not put more than about $25k more into the boat for all of this. And I'd like to have reliability (don't want some hi time engines that may need major overhauls, or break down at sea). I figured with outboards, I could buy new engines and run without much maintenance for five years.

Or maybe put two smaller diesels in.... can't afford 2 x 300hp diesels though...

[naturewaterboy]

I finally got some photos of the 34C Silverton posted to a website where anyone who's interested can view them.
Check out http://kkropf.photosite.com/

It's in need of cleanup, but the hull is solid - I replaced the wood in the stringers even though Silverton factory said the wood is not necessary for structural strength. The boat was very solid when we picked it up with a crane (2 times)- the local crane operators both said that it seemed like a very strong hull - some boats twist and bend when picked up. Anyhow, I'd like to make this boat move in the water economically, as it is the interior layout that I like.

[Frosty]

Your problem seems to be 3 things, expence and space to get 2 diesels in there and your draft requirments. Personally I think that 2 outboards will be swamped,-- then your in trouble.

I would spend a serious ammount of time trying to find a used pair of jets with motors that will fit in your space and are from a similar vessel. In comparison to refitting v drives of another flavour this would be reasonably straight forward. It would certainly give you what you want relability, shallow draft, and meet your space requirments and costs( possibly)

Once youve bought a a pair of jets and motors thats it,-- nothing else to buy.

[Willallison]

mmmm - jets would be a bit of a gamble, I think. True, you'd get the shallow draft, but the boat may be a complete dog at slower planing speeds. Having said that, I have little experience with them, so Jack may well be right...
Swamping outboards can easily be overcome by adding a 'snorkel' so that they draw their intake air from inside the boat.

[Frosty]

I just cant immagine any thing worse than 2 huge outboards on the back of a cruiser. I dont think that air intake is much of the problem, its more if the engines werent running, once the engine had a bit of water it is done ,-- I mean you cant work on it its too far down. plus being swamped every 10 seconds or the like. If you had to stop in bad seas for whatever i think your main concern would be the engines.
The engines would rot away in 6 months.

The proper way to do this is put back in what came out.

[Willallison]

Sorry Jack - I don't quite understand where you're coming from. Most bigger o/b's these days have their cowling sealed - with the exception of the air intake. Water shouldn't get inside. There are any number of 'big' boats designed to take outboards - their motors don't simply fall apart....
Having said that, I would tend to agree with you. In this case, if the cheap little displacement bay-cruiser concept is out, then I'd opt for putting in a pair of inboards. Problem is the overall budget here is so limited - and there's no point spending a fortune on this boat: it'd be money down the drain...

[Bob S.]

Well lets be honest - it'll be money down the drain no matter what Keith decides to do as this hull is getting close to obscurity and little value unless it is a mint one owner with original type power. This in spite of the fact that it is a sound, strong hull. I would opt for the outboards for several reasons, not the least of which they are easily transferred to another boat 2-3 years from now should another hull strike his fancy (or reality sets in).

[naturewaterboy]

Hmmmm....... that is a good point that outboards would be easy to remove and put on another boat, or sell, if this project would fail. An inboard installation will require transmissions, vdrives, new shafts, struts and rudders which would not be easily reused or would bring only a fraction of what I'd have to pay for them. So I'll put another mark in the "go outboards" column.

So, if I put a pair of 70 -150 hp outboards on the boat, I could probably run them just above idle and do 6 knots. If the seas got rough, I could run them harder ... would the boat be able to handle rough weather? Would I need more power for rough seas? I've been in 10-12 foot seas before, but not in an underpowered boat. Do I need a lot of power to handle rough seas? Again, if I don't get up on plane, that's ok, as long as I can handle some rough seas.

[Frosty]

Well after seeing the picture of the boat its not as big as I had thought, I dont know why but I was thinking of some thing bigger. I now completely change my mind and say that a couple of outboards would be ok. I would also say that In my opinion a pair of 150 might get that on the plane but wieght would be critical.



[QUOTE=Willallison;111521]Sorry Jack - I don't quite understand where you're coming from. Most bigger o/b's these days have their cowling sealed - with the exception of the air intake./QUOTE]

Will, Thats the point the engine needs air , and a lot of it, the carbs or intakes are under the cowling. Its the air intake I am concerned about.

[Willallison]

A pair of 150's will almost certainly not be big enough if you want to plane.
If not, there's no point in putting on anything bigger than a pair of, say 50hp. A boat like this will need maybe 20 or 30 hp to run at displacement hull speed (about 5 - 6 knots).
Putiing bigger motors on will only ensure that it uses more fuel.

[Bob S.]

A boat like this will need maybe 20 or 30 hp to run at displacement hull speed (about 5 - 6 knots).


I'll vouch for that. In my area we slap an 8-10 hp 4 stroke on fairly beefy fishing boats to troll for salmon. At WOT going with wind & current you can make 5-6 knots even with that little power. Likely that 50 hp would provide a similar speed for the 34 Silverton even into the wind. I wouldn't want to be in any real snot with those small engines though, at least not in a big, empty planning hull. You'd be sideways to the trough in a hurry and with not enough power to bring her around the bottom of your boat will likely be on the evening news.

[naturewaterboy]

So it sounds like I definitely don't want to put a pair of 50 hp on the boat then take it out in rough seas. If it had 2 150s on it though, wouldn't that be enough power to keep it from being pushed around - enough to keep it going where it was pointed? The boat could run at 6 knots with the engines loafing - unless things got sloppy, then crank the engines up and plow thru. Would this work?

[Bob S.]

Nobody intentionally goes out in rough seas (usually), problem is they get nasty once you are out there. The bigger power would provide a means to make safe harbor at a reasonable clip, to keep your nose in the direction you want when bad things overtake you or safely in the slot when you need to run an inlet. No law says you can't shut one of them down and putt along at 1500 rpm on the other one when conditions allow. I'd set it up to run as a properly powered boat. Even the biggest 4 strokes hardly use any gas at displacement speeds but the power will be there when you need it.

[Frosty]

I agree with Bob you dont want to be in a trough and not be able to get the stern round, and in a bit of wind you wont and thats frightening stuff. I think a pair of 150's with 17 pitch might get it onto the plane but you will have power to save your life.

Here in Thailand they have huge speed boats probably a similar shape made from ply. They are about 30 feet and carry 60 pax to islands such as PiPi ( where they made the Beach movie) these monsters use 3 or 4 Yamaha 200 hp and handle rought stuff ,--infact they are called upon occasionally to go out to rescue.

[Willallison]

Unless we know the boats approx weight (preferably withoiut the old diesels) then there's no way of knowing if it would get on the plane with a pair of 150's. If it's not too heavy, it might, just - though remember it originally had a pair of 260hp inboards, which were considered underpowered....

To suggest that you need 300hp to simply 'bring the stern around' of a displacement boat is ridiculous. If it needs 30hp to putt along with, then 100hp ought to be more than enough to push it 'around'. The problem though - regardless of engine size - will be ensuring that the props remain under water.

[Frosty]

Yes Will the boats wieght would be of great assistance to move forward on this. However I dont think any one was saying that 300 hp was required just to push the nose around, but if you got it then you got it. On the other hand I would not under estimate the amount of power required to quickly get the nose around. If you had the misfortune to find yourself side on to the sea and you had 3-4 seconds before the next wave I would gas those 150's

In a wind and with that boats wind resistance the bow will be all over the place and if left to drift would probably present her stern to the sea. If for no other reason that the props would offer drag.

For such conditions as these I carry a sea anchor, but doubt and hope I will never use it as I genarally island hop for the reasons stated above . My power cat has 500 HP and at zero speed I could not bring her round in 5 seconds

[Bob S.]

Its the same problem we all wrestle with . . . trying to make one boat do a variety of contradictory things. In this case the owner needs the attributes of a lake-bound shallow water houseboat along with the ability to range far offshore and be safe in poor conditions. To complicate things instead of starting from a blank sheet of paper these characteristics are to be coaxed out of an old beamy powerboat hull. Makes for great conversation . . .

[Pericles]

This site may help you. http://armstrongnautical.com/brackets.htm

Pericles

[naturewaterboy]

Yes, I've talked to Armstrong and a couple of other manufacturers at the boat shows about building a custom bracket for the boat - probably cost about $5k. Thought about finding a good welder and building my own, but probably just let Armstrong or Gill or Stainless build one. I think it was Armstrong that was the most willing to build one for this boat - but none of them could tell me much about any brackets they built for similar boats.

[Bob S.]

Your best bracket for that boat would be a Porta type. It would give you up to 24" of vertical lift which would provide the flexibility you'll need. Way up for shallow running and way down for offshore bite. Its not a buoyancy type of bracket which I don't think you'll benefit from considering all the weight that has been removed from the aft end of the boat. You are going to be light back there anyway IMO. Easier install than an Armstrong type as well.

http://www.portaproducts.com/bracket.html

Ask for Scott, they are in Edgewater

Put a twin engine, 26" setback Porta Bracket on that thing with a pair of 175 Suzuki 4 strokes and you will be happy

[naturewaterboy]

Here is some more info on the boat - I believe the boat weighs about 10,000 to 12,000 lbs. without engines. As you can see from the picture in an earlier post, the hull sticks out of the water and will catch a lot of wind. I have a rpm speed table from another 34C Silverton with inboard gas engines - I'll paste it below - it has 7 columns and it's hard to read when pasted, I'll attach an excel spreadsheet also:

1978 34' Silverton Sedan w/ 1986 270 hp Crusaders spinning 17"x19.5" 3 blade props
Full fuel, 1 person onbard with 100 pounds of gear
INTO THE WIND AT 10 MPH

RPM's gph - port gph starboard Total Speed
Knots Miles per hr Miles per gallons
1000 1.5 1.75 3.25 4.6 5.2 1.6
1500 2.5 3 5.5 6.4 7.3 1.3
1600 3 3.5 6.5 6.9 7.9 1.2
1800 3.25 4 7.25 7.1 8.1 1.1
2000 4 5 9 7.1 8.1 0.9
2200 5 5.5 10.5 7.6 8.7 0.8
2400 6 7 13 8.9 10.1 0.8
2600 6.5 8 14.5 10.4 11.9 0.8
2800 8 9.5 17.5 11.4 13.0 0.7
3000 9 10 19 13 14.8 0.8
3200 10 11 21 14.4 16.4 0.8
3300 11 13 24 15.5 17.7 0.7
3400 11 15 26 16.7 19.0 0.7
3600 12 18 30 17.8 20.3 0.7
3800 15 18 33 20 22.8 0.7
4000 16 20 36 22 25.1 0.7
4600/4400 20 22 42 24 27.4 0.7

WITH THE WIND, AGAINST THE CURRENT
3200 9 11 20 15.8 18.0 0.9
3400 11 14 25 17.1 19.5 0.8

[Bob S.]

see post #40

[naturewaterboy]

I've decided for now that I'm going to put on a bracket and hang a pair of 250 or 300hp 30" shaft outboards on it. I am going to look for a pair of used engines, and have one of the manufacturers design a bracket with flotation. I can then experiment with fuel and water tank placement to balance the boat properly.

[Willallison]

I think you should do a simple weights and moments calc to establish just how far out of balance you're likely to be.
Take the weight of the inboard engines and gearboxes and multiply by the distance aft from the bow. This gives you the moments.
Do the same for the outboards, including the bracket. It'll only be rough because you are ignoring the shafts etc, but it will give you a rough idea of what you're up against.

Trying to ballance the boat using tankage is best avoided if you can, as these (quite obviously) change depending on how full they are. Batteries and other permanent things are better if they're sufficient.

[Frosty]

You should have no problem balancing the wieght after all you now have an empty engine room . Store your stuff there ropes ,spare anchors, beer, jerry cans, whatever, Paint tins, and beer. You could also put in a fresh water tank , thats handy!!!

[Willallison]

You should have no problem balancing the wieght after all you now have an empty engine room . Store your stuff there ropes ,spare anchors, beer, jerry cans, whatever, Paint tins, and beer. You could also put in a fresh water tank , thats handy!!!

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

[naturewaterboy]

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?

[Willallison]

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

[naturewaterboy]

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

[Willallison]

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

[naturewaterboy]

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

[naturewaterboy]

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! :D I do have a lot of room belowdecks to move fuel, water, batteries, etc. if I need to adjust CG.

What do you think?

[naturewaterboy]

OooPS! I meant Willallison....

[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.

[naturewaterboy]

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!:D

[naturewaterboy]

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:mad: 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.

[Frosty]

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.

[Willallison]

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.


:confused: 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.

[Frosty]

:

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!
.


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.

[naturewaterboy]

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.

[naturewaterboy]

getting closer to putting this boat in the water. I have purchased two 2006 Mercury Optimax (HPDI) 225 hp 2 strokes, have a Stainless Marine 4 engine bracket almost ready to mount on the transom that I reinforced with 6 layers of 3/8" Okume marine ply with glass mat between for a total transom thickness of 3" now. The outboards have 30" shafts, I've set the bracket so the engines sit 33" above the bottom of the boat. Gonna find out soon if this works.

If the boat was built with 270 hp inboards, what can I expect to get performance wise out of 225hp outboards? I've shed around 1000 lbs. or more? Anyone have a guess on performance?

[Frosty]

What do you reckon your TOTAL weight will come in at? Fuel N all.

Have you chosen some props yet if so what?

[naturewaterboy]

I'm guessing it will have about 13,000 lbs. displacement with full fuel. I haven't thought about props yet. I hope I can try several props - any suggestions?