46 Convertible refit begins

[Walter P]

The thread krush posted link to is the one I own. Right now I am only getting 2150 RPM on port and 2275 on starboard while reaching 27kts. So when we fix whatever is holding back port I should reach the 29 kts Dave was getting. We don’t carry a bunch of junk on the boat so not sure how it is 4 kts off of what it reached new. Maybe that run was with empty water, holding, and aux tanks.

Anyway if you go in the 600-700HP range you get an idea on performance. As for modification to the hull on the HP the only one I am aware of is the “ cut down keel”. Don’t know if hull was built this way or modified after.

One of my friends bought a brand new 46 HP sometime in the mid 80's and I remember talking about the
differences with the hull. Along with the keel redesign, the hull was also strengthened to compensate
for the additional power. We were out playing with his new boat and did well over 30K averaged on
several reciprocal runs. This was in Panama City Beach, Fla in the fall or winter as I remember it was
cool at the time. That was quite fast for that time. Today the demand seems to be for 40k or more.

Walt

 

[krush]

Dig through the forum posts and I'm sure the information is there on HP. Dave will probably know as well. I recall hearing something about slightly larger shafts? And maybe a little bit more beef in the stringers?

 

[madhatter1]

Link to useful thread pertaining to this discussion.

https://www.samsmarine.com/forums/showthread.php?13630-1050-MANS-in-a-46C

 

[Mick c]

Thanks everyone for there input. Seems like there is a few different opinions in regards to this. I will keep digging for info but at the end of the day I’m tending toward the suggestion of just pulling back the sticks. Mick

 

[q240z]

So...Mick, if you get a second, could you let us know the rationale for going with the 92 series engines rather than just rebuilding the 871s you've already got? Seems like a lot of extra work and $$$ for not a lot of additional BANG, if you know what I mean.

Cheers,
Q

 

[Mick c]

Just to put some context around our plans our estimate is that we have to rebuild either the 71 s or the 92 s. We have got. Guy that will do this for about 7k each and I suppose by time we do the turbos and all the other auxiliary items then they will prob be about 10 k AU each. We are purchasing the second hand engines that are currently in a 50 ft boat and have only done 1600 hrs. We still intend to rebuild these as well. These engines are 1998 model 625 hp and electronic controlled and for 25 k au we get everything from the controls to the props. Our 871 are only 430 hp est so there is a significant increase in Hp . We removed the engines today and I will post some photos and I will try and post a video if that’s possible. Based on the info what everyone’s thoughts on the rational ? Regards Mick

 

[Mick c]

So today we removed the engines and generator and was able to pressure wash the engine room. I tried to post a video but with no success ? Is it possible to post vids on this forum? I will post some photos. Regards Michael

 

[Mick c]

More photos.

 

[Mick c]

2 more.

 

[Dazed338]

Nice, love the beam setup

 

[q240z]

Looks great, Mick.

Presumably, rebuilds will cost roughly the same whether it's your 871s or 692s. So the 25 k au you mentioned is buying the added HP. IIRC, you mentioned before that you'd be inclined to just pull back the sticks, which I took to mean you tend to cruise or don't mind cruising at relatively low speed. One of the other fellas here mentioned the 92s have issues the 71s don't have, which is why I thought rebuilding the 871s seemed like a better approach, especially for a lower speed cruiser.

Cheers,
Q

 

[krush]

Based on the info what everyone’s thoughts on the rational ? Regards Mick

Just my opinion, but I think you are overpaying for the 6v92 detroits.

 

[Mick c]

Looks great, Mick.Presumably, rebuilds will cost roughly the same whether it's your 871s or 692s. So the 25 k au you mentioned is buying the added HP. IIRC, you mentioned before that you'd be inclined to just pull back the sticks, which I took to mean you tend to cruise or don't mind cruising at relatively low speed. One of the other fellas here mentioned the 92s have issues the 71s don't have, which is why I thought rebuilding the 871s seemed like a better approach, especially for a lower speed cruiser.Cheers,Q

I hear what you guys are saying and yes it is a little contradictory to what I have mentioned in some of my last posts but unfortunately we are rev heads at heart and we always tend to go in that direction eventually. Our prop guy has done the calcs on the boat/gearbox’s and prop combo and suggests that we most likely what we wil achieve around 26-28 knots which I wouldn’t regard as outrageously powerful. Anyhow I do appreciate all the feedback we get good or bad. Mick

 

[osprey]

Hi, I have a 1982 46C HP, 8V92Ti 650 hp. Allison MH transmission weight 20 tons. Max rpm 2300. At WOT with half fuel she will do 27knots. Although very smooth, I feel for the old girl. With FULL fuel ( 2400 liter ); if I generally cruise at 20 knots, Do a little trolling and Bring it up to full throttle for short times I can go about 200 miles. If I generally do 8 to 9 knots, a little fishing and bring it up to WOT a couple times for about 15 min, I can go well over 400 miles. I believe that the best reason for more HP is for when you need the boat to preform, not just to go Fast. The weather here tends to change without notice. It is nice to have good response when the seas become nasty. In your apparent situation, I would keep it simple, stay with the smaller 71's. If you really want the extra work, cost and HP, Change your budget and get modern 4 strokes. The higher HP Detroit's require a lot of TLC to stay clean. I recently rebuilt my starboard engine with the aid of a manual, very straight forward. Even if they look good, buy new exhaust manifolds and risers. Rebuild the injectors. I also bought new turbos and have rebuilt one of the old ones for a spare. the hardest part was the weight of the parts and that I am on a mooring. All the best, Have fun.

Pete

 

[Mick c]

Hi, I have a 1982 46C HP, 8V92Ti 650 hp. Allison MH transmission weight 20 tons. Max rpm 2300. At WOT with half fuel she will do 27knots. Although very smooth, I feel for the old girl. With FULL fuel ( 2400 liter ); if I generally cruise at 20 knots, Do a little trolling and Bring it up to full throttle for short times I can go about 200 miles. If I generally do 8 to 9 knots, a little fishing and bring it up to WOT a couple times for about 15 min, I can go well over 400 miles. I believe that the best reason for more HP is for when you need the boat to preform, not just to go Fast. The weather here tends to change without notice. It is nice to have good response when the seas become nasty. In your apparent situation, I would keep it simple, stay with the smaller 71's. If you really want the extra work, cost and HP, Change your budget and get modern 4 strokes. The higher HP Detroit's require a lot of TLC to stay clean. I recently rebuilt my starboard engine with the aid of a manual, very straight forward. Even if they look good, buy new exhaust manifolds and risers. Rebuild the injectors. I also bought new turbos and have rebuilt one of the old ones for a spare. the hardest part was the weight of the parts and that I am on a mooring. All the best, Have fun.Pete

Thanks Pete, sounds like my prop guy estimated correctly when he said the boat should do about 26-28 knots based on the performance of your boat. The big difference for me is that these particular 92 are electronic over the old 71s and yes I would love to fit a couple of Cummins into the boat but that will add at least 150 k au to the project. Maybe later. I would rather be cruising at 16 odd knots with the 92 than ringing the neck out of the 71 s ?. What props are you running with what shaft size.?

 

[Mick c]

Nice, love the beam setup

Thanks for that. Spent a lot of time planning the removal of the engines. The hired gantry was extremely adjustable and modular. It helped having the machinery skates under the gantry for sideways movement as well. Had about 0 clearance out the door but all in all went to plan. Mick

 

[osprey]

Hi Mike, I have 2.5 inch shaft and am running 28 x 26 four bladed props. The original 3 blade props 28 x 27 where also pretty good, but only could get about 25knots. I changed them for my spare set because the 3 blade props needed to be tuned.

Have fun,

Pete

 

[osprey]

A thought on Speed, My 46C feel good hull speed is about 8 to 9 knots at about 850 rpm. More rpm and I start pushing water. At which point I would push up to 2100 rpm, get her on the plan and will do a little more then 22 knots then slowly retard engines to settle into about 1850 rpm cruising at about 20knots. On a good day I can throttle back to a little over 1650 rpm and she will glide along at 17knots. any slower or if there's a bit of wave action, she will fall off the plane. 11 thru 16knots are not speeds I can use. Pete

 

[Mick c]

IT really sounds like every boat and every engine and prop combo has different outcomes. I guess we won’t know till we get it back in the water. Thanks M

 

[jim rosenthal]

I think the reason that people are questioning your plan to go with 92 series DDs is that theya re very intolerant of overheating- the silicone seals at the end of the liners can leak if they are run too hot, and apparently if the cooling system isn't up to snuff they WILL run hot. And when they leak, it is easy to really damage the engine, and it all happens pretty fast.

It seems to me (from half a world away, granted) that you should either keep our 8-71s, or look for modern 4-stroke engines, which would preferably be Cummins M11s. I think the 92 series will cost you a lot, and not get you that much additional performance...

Of course, it's easy to give advice, and the free advice here, including mine, will be worth what you pay for it. Maybe the best way to proceed is to clean up the boat and do all the updates and keep thinking about what engines you want to put in. After all, you have a lot of prep work to do before anything goes back in, so maybe you don't have to make a decision right away.