View Full Version : Outdrive questions
I'm building a 28 foot runabout boat and am still working on the hull. I won't be righting the hull for a while but am still anxious to explore the steps that follow. The one thing I don't understand much are the dynamics of the outdrive. In my particular case I will have an inboard between two cockpits - essentially the middle of the boat with the shaft driving through the floor. What kind of outdrive am I to look for? and what considerations do I need to make?
brad9798
10-24-2002, 08:38 AM
Unless you place your engine just inside your transom, you won't be having an 'outdrive.'
Sounds like a straight shaft inboard application to me?!? Or, am I missing something?
Brad
kilo...it is an inboard...you'll need, aft of the engine, a transmission, a cutless bearing, a shaft running through a shaft-log to exit the hull, one or two shaft struts, a prop and a rudder. You'll also need to provide for the engine's respiration, fresh air available to breathe, and a means of exhaust. You'll need a starter, a battery, a fuel tank and line.
rkrough
10-24-2002, 10:34 AM
maybe this will help
http://www.imagestation.com/picture/sraid36/p07aafd282c464042ed85263edc9633d5/fd20fa95.jpg
Bill Berrisford
10-24-2002, 10:59 AM
Kilo-
Mercruiser manufactures units with drive shaft extentions, allowing the engine to be mounted well forward of the outdrive (common in racing boats).
-Bill
Ok.. I see.. That diagram cleared up many questions. So how do I determine what kind of transmission to install?
brad9798
10-24-2002, 02:30 PM
Many times, when you buy new or used marine engine, it will come with a transmission, or at least with an option of one.
This is something to discuss when you decide upon a particular engine.
For example, if you are going to use a 283, or 350, or 327 Chevy engine, you could easily us one of the paragon transmissions with the appropriate reduction gear ... typically 1.5-2.5:1 in ratio.
For your boat, more for speed, 1.5:1 would work just fine.
Brad
What does 1:5:1 denote? These transmissions I assume come only with forward, neutral, and reverse, is this correct? And if this is the case, why is it that there is only one forward gear? Is there such a thing as a 3 or 4 speed? Or does hydrodynamics prohibit that?
Kilo 1.5.1 is 1 1/2 turns of the motor to 1 turn of the trans.boats use tork to move through the water unlike a car going down a road (uses horsepower)so the more rpm the more tork.
good luck JLM smile.gif
Ok.. so correct me if I'm wrong - the higher the ratio from motor to trans, the more torque initially and lower top speed. So how does one determine their top speed? By using the redline determined by the manufacturer?
Interesting surely. I'm hoping to see 50 mph out of my boat and need to figure out how to make it happen inside my budget. I saw a GM 4.3 vortec on ebay for 15k. Could a 4.3 achieve this my goal? and what are your thought on the $1500 - high/low?
JimConlin
10-25-2002, 12:30 AM
Are we being trolled?
Hey I’m an amateur which I believe my posts manifestly suggest. I’ll kindly disregard the comment. Good luck on your Spark daysailer.
rkrough
10-25-2002, 08:28 AM
to get 50mph in a 28 foot boat the 4.3 won't do it unless the boat is very light. You need to determine what the completed boat will weigh fully loaded with fuel,pasengers etc
(displacement) and divide the result by 10 to get in the ballpark for required HP at 50mph (example: 3000# boat would need 300hp to get to 50mph). There was an article in National Fisherman a few years ago that dealt with repowering and the figure I gave came from a graph related to the article.
rich
I had a Vortec 4.3 in an Astro van, and it was one fine engine. Fast and plenty of torque, with reasonable mileage. Stick 2 of 'em in her, and you'll have a "runamuck" instead of a runabout!
brad9798
10-25-2002, 08:56 AM
50mph?
You better be considering a 454, or a modified 350 block, then.
Speaking from a 28' runabout in my marina, she has a 454 (330hp) 'factory' engine, and she will run 50mph with a bravo three outdrive (not a straight inboard).
50mph is deceptively fast, and it takes a lot of power to move MOST 28-footers at that speed.
Perhaps you could pick up older 327 or 350 block- you'll hit into the 40-mph range with those.
The power increase required to raise speed dramatically increases the faster you go- It becomes just as much a function of the hull.
Take a 20-year-old 40 Scarab in my harbor with twin BUILT-UP 454 blocks. It runs at a good 65mph.
Then, there's the new 47' Fountain with LESS hp (although they are 502 blocks), and it runs 75mph ... and it's 5 feet longer. But that stepped hull is sweet.
You get my point- it's not all about power.
Brad
mhoffman
10-25-2002, 09:04 AM
I am just finishing a 21 foot runabout with engine located forward of the cockpit. I looked for a while and decided on a new engine vice used one. Bought a freshwater cooled, fuel injected 350 with transmission for under 10k. Shop around- you can do a lot better than 15k for a 4.3...
Keep reading and asking questions- applying what you have learned to do it right the first time is a lot more satisfying experience than doing it wrong the first time and then having to learn how to do it properly...
Best-
Matt
chesterm
10-25-2002, 01:32 PM
For a 28 footer a big block is the only way to go. Don't even consider a small block if you have desires to break 40 m.p.h. regardless of advertised horsepower ratings. It is the torque more than the horse power that will move a boat that size.
Get some books or resource information on engine installations. Jim's previous "troll" inquiry is understandable, it's uncommon for someone to be as far in to a 28 foot project and know so little about the remainder of the project. I just finished a straight inboard installation in my 23 foot runabout with an engine placed between cockpits. I'd be happy to be a sounding board for you. email direct if you care to. regards, mitchell in CT
Sorry guys - I meant 1.5K -
I did see a 454 for 4k and that was out of my budget - Grrr.
Here's my current state:
http://www.batchlabs.com/temp/boat.jpg
notice the VERY confined space I'm dealing with.
cheak out your local junkyards and find a used 454 or 350 and have it rebuilt. i used to buy 350s for around 200 to 450 bucks rebuild for under a 1000 but if your going to build the motor up make sure you get a 350 with 4bolt mains (4bolts on the center crank bearings)instead of 2.then just buy a trany and bolt it on and your good to go.
good luck JLM :D
ion barnes
10-30-2002, 04:15 AM
Look for a second hand marine store, you should be able to find the motor/trany system of your choice plus some of the other parts as well,BUT beware of junk parts like shaft tubes with wear marks almost thru the gland! Some time spent looking for used at a resonable price is better than paying the extreme for brand new. And it can be a lot of fun. So who is the designer of your project?
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