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The ins and outs of inboards

Old 12-06-2019, 10:06 AM
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Default The ins and outs of inboards

I am hoping to get a used 30' or so sport fisherman style boat here in a couple of years and part of the process is educating myself as much as I can on everything that goes into them. I've owned boats for years, rebuilt my 18' CC from the hull up and have done extensive engine and transmission work in the past so a lot of the knowledge transfers. But learning from those with real world experience is a guide that can't be beat.
Reading threads here on THT while also looking at used boats for sale here as well as on boat trader has been quite eye opening but inboard drivetrains are not ever listed.

I did find this very useful diagram and searched the site but didn't come across much.


Which of these configurations would be preferred?
As for brands, is one brand better than the other?
Are they something that just works?
How are the prop shafts typically sealed? O-Rings or packing? When I was involved in a retrofit (electronics) of a ship several years ago, I saw how it was done on there; however, they let a trickle of water flow in all the time while running.
Are there sealed bearings on the prop shaft struts?

I know that posters here have run a lot of these and should have their likes and dislikes.
Any insights?
Old 12-06-2019, 10:20 AM
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what is the difference with a co-axial in-line drive?

ps. you never followed up your thread: Protecting yourself online.
Old 12-06-2019, 10:26 AM
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Each configuration you show would be specific to a hull / displacement / configuration or type of boat. Pick your boat based upon needs then select power options!
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Old 12-06-2019, 10:35 AM
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The configuration is going to depend on how the engines fit into the engine room. Mostly you will see V drives which is what I have on my boat. The engines face the back of the boat with transmissions on the back of the engine with a shaft that goes under the engine. The transmissions are counterrotating.

Brands are going to vary but Hurth and ZF are pretty popular transmissions.

Brands
Gas: Crusader, Mercruiser, Volvo Penta - Crusader is by far the best, Mercruiser is fine and Volvo is good but as with everything Volvo parts are expensive.
Diesel: Cummings, Cat, MAN, MTU, Volvo, John Deer, Perkins - mostly you will see Cat, Cummings and Volvo if you buy diesel its about engine model more so than manufacture. The marine diesel industry has a bad habit of taking blocks designed for 250HP and putting larger and larger turbos on them until they have 400HP and have problems.

The prop shaft is sealed via stuffing box which requires the cord to be changed periodicity, needs to be adjusted and drips by design or dripless shaft seals which is what I have

Once the shaft exits the boat it is supported by a strut that has a cutlass bearing in it. The bearings need to be periodically changed.

Props are big bucks on inboards. $1000+ each side

Grounding can cause a lot more damage

Any other questions?
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Old 12-06-2019, 10:48 AM
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Vdrive transmissions tend to be more expensive to replace and repair, and can be more difficult to service based on access, complexity, etc.. Vdrives are common in express cruisers with aft cabins, since they enable the engine to be further aft leaving room for the aft cabin. However, boats with an aft cabin usually have a smaller cockpit. So for express cruisers around 30 feet, if you want an aft cabin so you can sleep more than 2, you are probably looking at vdrives. If you don't need the aft cabin for additional sleeping capacity, then an inline setup (engines further forward) with a larger cockpit but no aft cabin is the way to go. Sport Fish in the 30' range are almost always inline drives, as there is no aft cabin, and the engine can be further forward leaving room for the straight shaft. I have also heard that vdrives sap more power than a straight inline drive, which makes sense to me, but I have never really researched that, it might not be enough to really matter. Personally, I would find the boat that I wanted, and would take either a vdrive or inline setup, the differences aren't large enough to outweigh the other factors in finding a boat I would want to buy such as condition, maintenance, survey results, price, layout, etc.. I have a Tiara 2700 express, with twin inline inboards, for me the bigger cockpit is more important than an aft cabin.
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Old 12-06-2019, 10:58 AM
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Originally Posted by Lorne Greene View Post
Props are big bucks on inboards. $1000+ each side
try $3k + average per prop on 40' ish foot inboard boats... just for the prop... plus costs to pitch, balance, etc.
Old 12-06-2019, 11:36 AM
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Standard in-line need a higher deck to accommodate the height of the engine, but they are very simple and straightforward installations, usually with plenty of room for maintenance.

V-Drives can pack the engines toward the aft of the boat, but also might pack them so tight that maintenance can be a real bear. Make it tough to get to things like raw water pumps to replace the impeller.

Prop struts have cutlass bearings which are lubricated with the water going through them. They do need to be replaced eventually, but can last for very, very long time especially if the props and shafts are balanced and aligned properly.

The prop shaft seals can be the "old school" packing that need to be adjusted or replaced periodically. They should drip occasionally because that's what lubricates them. There are also dripless seals that are very popular now but they are more expensive to install. They do not drip though, so the result can be a very dry bilge.
Old 12-06-2019, 11:38 AM
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Originally Posted by mystery View Post
try $3k + average per prop on 40' ish foot inboard boats... just for the prop... plus costs to pitch, balance, etc.
The OP is talking about a 30' vessel, so I think the prior estimate is closer to reality (although maybe just a tad low).
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Old 12-06-2019, 12:23 PM
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With your skills, there are plenty of older, well-maintained straight-shaft boats in that size that will serve you well, for little investment/risk. If you decide it isn't for you, you won't lose much if any money. Look for older Chris Craft, Bertam, even Hatteras, you can find them in very good shape, cheap. Not a huge demand for them. Prices are low enough you find many on Craigslist, use a search app like https://www.searchtempest.com/ I've found several with new engines, asking less than the engines had to cost. If you have to restore one you will never get the money back, only the enjoyment. I budgeted and still want to drop in a pair of 383 Strokers, but this one only has 500-hours on the engines. Maybe in a few years. I'll never sell it for what the engines will cost, but will have my perfect boat.

I'm replacing my packing box hoses, thought about going to dripless while the packing boxes are in-hand, but I think I'll stick with the more rugged packing boxes. I really liked the dripless in my last boat, but a significant weak point in hull integrity. Dripless use a face-seal that have some weaknesses, or a lip seal that is exactly the lip seal used in engine front and rear main seals, transmission shaft seals, pump shaft seals, etc. Losing water flow or damaging/neglecting the bellows can cause your boat to sink, while when old packing boxes fail you get a drip or trickle at worst.

I like simple and reliable, especially far from land, or on beautiful summer days. An old, carbureted Chevy 350 isn't super powerful or super efficient on fuel, but it will cost you very little to operate, maintain, even replace.
.

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Old 12-06-2019, 02:10 PM
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Straight shaft with down angle transmission. most are 8 degree. Older (vintage) transmissions didn't have the down angle. I think every modern design has it. It helps keep your engine level at rest. All engines are tilted back when running. Stay away from v-drives. Getting to the packing gland is usually tough with v-drives. Take a look at an installation and you will see what I mean. Also, for inboards, pay special attention to the exhaust angle. Measure the height of the elbows above the static water line. It should be 12 inches or so, otherwise water will back flow up the pipe.
Old 12-06-2019, 02:35 PM
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There is something about the simplicity of the straight shaft drive that is appealing. You probably lose in performance from the inboard shaft angle what you would lose in the gear trains of a V or stern drive. And on an older boat, there's just that much less to go wrong. Consider too that any boat with a prop under the hull as compared to an outboard or sterndrive is going to have more hydraulic vibration and noise transferred up through the bottom.
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Old 12-06-2019, 02:45 PM
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Originally Posted by saltwaters View Post
Straight shaft with down angle transmission. most are 8 degree. Older (vintage) transmissions didn't have the down angle. I think every modern design has it. It helps keep your engine level at rest. All engines are tilted back when running. Stay away from v-drives. Getting to the packing gland is usually tough with v-drives. Take a look at an installation and you will see what I mean. Also, for inboards, pay special attention to the exhaust angle. Measure the height of the elbows above the static water line. It should be 12 inches or so, otherwise water will back flow up the pipe.
I'm planning on a down-angle when I repower some day, how much if any efficiency loss to that angle drive? I've only run straight drives on my inboards.
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Old 12-06-2019, 02:49 PM
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Originally Posted by 706jim View Post
There is something about the simplicity of the straight shaft drive that is appealing. You probably lose in performance from the inboard shaft angle what you would lose in the gear trains of a V or stern drive. And on an older boat, there's just that much less to go wrong. Consider too that any boat with a prop under the hull as compared to an outboard or sterndrive is going to have more hydraulic vibration and noise transferred up through the bottom.
I've not heard before of noticeable noise or vibration from an inboard prop, unless the prop tips are too close to the hull, which brings other problems. That will be something I'll have to deal with when I re-engine and want bigger props. My last boat had semi-tunnelled props, and the tips were pretty close to the tunnels, maybe the physics is different in a tunnel.
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Old 12-06-2019, 02:52 PM
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Originally Posted by km1125 View Post
The OP is talking about a 30' vessel, so I think the prior estimate is closer to reality (although maybe just a tad low).
I have a 32 foot boat and just paid 1k each used prop.
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Old 12-06-2019, 02:54 PM
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Another advantage of straight shafts vs vdrives is with the engines further foward and low in the hull, there is better weight distribution and a low center of gravity. An inboard with a deep V and straight shafts will be the best running when in rough conditions. V drives, I/O's and outboards stack alot of weight all into the back of the boat, they usually don't run as nice when the conditions get "sporty". Of course outboards and I/Os are faster, more fuel efficient and less likely to have huge repair bills if you kiss some rocks. In regards to maintenance, outboard fans tend to insist a modern outboard is cheaper and easier to maintain, but that isn't the case. Assuming good access (some boats DO NOT have good access), a Chevy small block or big block by Mercruiser or Crusader is tough to beat in terms of ease of maintenance, easy to find and cheap parts. Will eat gas though. Diesel inboards are another story, longer life, more reliable IF maintained well, better efficiency, but more expensive up front and VERY expensive parts and maintenance. All trade offs, no such thing as a perfect boat!
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Old 12-06-2019, 08:57 PM
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Thanks. I appreciate the input.
Old 12-07-2019, 12:42 AM
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I like my inboard. water cooled drip less shaft. You do have to mind your depth though
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Old 12-07-2019, 08:25 AM
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K.I.S.S.

To me that means straight inboards with prop pockets. Low maintenance and easy to work on. On my Sabre I can do engine work while out of the weather even have A/C if I want.
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Old 12-07-2019, 08:50 AM
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I gravitated to a single diesel with prop tunnel, drop center gear (no down angle) and bow thruster. Been very happy with it. Not as fast as outboards, but quite fuel efficient and nice running. Standard packing gland with modern packing has been super reliable for 2700hrs. Cutlass bearings (two, due to 10' shaft) are still in good shape after 2700hrs.

Twins do give you redundancy should one engine have a problem, but the downsides are they tend to be crammed in and hard to work on, and you have to deal with 2X everything (maintenance and problems).
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Old 12-07-2019, 09:29 AM
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Originally Posted by mash_33316 View Post
Another advantage of straight shafts vs vdrives is with the engines further foward and low in the hull, there is better weight distribution and a low center of gravity. An inboard with a deep V and straight shafts will be the best running when in rough conditions. V drives, I/O's and outboards stack alot of weight all into the back of the boat, they usually don't run as nice when the conditions get "sporty". Of course outboards and I/Os are faster, more fuel efficient and less likely to have huge repair bills if you kiss some rocks. In regards to maintenance, outboard fans tend to insist a modern outboard is cheaper and easier to maintain, but that isn't the case. Assuming good access (some boats DO NOT have good access), a Chevy small block or big block by Mercruiser or Crusader is tough to beat in terms of ease of maintenance, easy to find and cheap parts. Will eat gas though. Diesel inboards are another story, longer life, more reliable IF maintained well, better efficiency, but more expensive up front and VERY expensive parts and maintenance. All trade offs, no such thing as a perfect boat!

agreed and if I could find a boat in the 20' size range with a straight inboard with GM V8 small block I for sure would buy one (like the Shamrock fishing boats with the keel drive) as long as engine access was good. A straight inboard, with closed cooling, is the least expensive, simplest drive package you can find. Not fast, maybe requires more fiddling than outboards, but repairs and parts prices are so much cheaper. Frankly I take great exception to the price of outboard engine parts. Everyone is convinced they "must" have these high priced engines, because inboards are too slow and require too much maintenance. Till you have to repower....then the dollar signs are staring you right in the face!

I always found it ironic that outboard fans, criticize I/Os because they use aluminum construction in salt water, well what do you think you are doing when you tilt those expensive outboards, out of salt water? Exposing salt water soaked internals to air, making internal corrosion do its work, which is why after 10 years they are on borrowed time due to internal corrosion, whereas with inboards, you can protect the engine, and most of the exhaust system with full closed cooling.

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