Repower Questions

[Timeless72]

Hope everyone had a great season! A local shop brought up the repower conversation. While we are quite a ways out from anything like that, it had me thinking a bit. These may be simple questions to some, so apologies in advance.

With a new engine being smaller and lighter, what are the weight savings implications? For example, 2k pounds savings will change center of gravity. On a boat already top heavy, 48 Yachtfish, does that mean we are offsetting with ballast or able to truly save the weight? Has anyone noticed this after repowering?

Also, based on how we use the boat, I would be looking for performance gains. Increased cruising speed. With a model on the rare side, low production numbers, how do we determine performance requirements? Specifically, I would love to see that boat capable of 18 kt cruise. I am not aware of this hull being used in other models. Any experience, please share. Thanks!

 

[warrenw]

Hope everyone had a great season! A local shop brought up the repower conversation. While we are quite a ways out from anything like that, it had me thinking a bit. These may be simple questions to some, so apologies in advance.

With a new engine being smaller and lighter, what are the weight savings implications? For example, 2k pounds savings will change center of gravity. On a boat already top heavy, 48 Yachtfish, does that mean we are offsetting with ballast or able to truly save the weight? Has anyone noticed this after repowering?

Also, based on how we use the boat, I would be looking for performance gains. Increased cruising speed. With a model on the rare side, low production numbers, how do we determine performance requirements? Specifically, I would love to see that boat capable of 18 kt cruise. I am not aware of this hull being used in other models. Any experience, please share. Thanks!

Having just repowered let me through some of my thoughts in here:
1) Weight. I was sensitive to this issue as well - how much weight can you reasonably remove before stability is impacted? A few years back I recall reading a story of a Chris-Craft 57' Constellation that was repowered from the original 8V71Ns to modern Yanmars. It was a charter boat and apparently the weight reduction was significant enough that it failed the stability tests. When I repowered a couple of guys recommended I replace the VT8-370Ms with Caterpillar 3208TAs as they were the closest match in terms of hp, size and weight that I would be able to reasonably find. The Cummins with gears were 2775lbs each and the Caterpillar 3208TAs with the ZF gears I installed are around 2080lbs, so I still lost close to 1400lbs total. The nice thing about this weight loss is that it offset the weight of large waterlift mufflers, added batteries, inverter, larger waste tanks, toolboxes and a pile of other stuff that I added to the pump room and engine room.
2) Calculating Performance. I used the calculator on BoatDiesel. Use your existing sea trial numbers to figure out the hull constants and then start plugging in new engine scenarios. It's reasonably accurate based on my experience - I did not need to repitch the props I selected. If you are getting a yard to repower they should work with the engine distributor to calculate gearing, shaft diameter and prop dimensions and give you projected cruise and WOT speeds.

 

[SeaEric]

Having just repowered let me through some of my thoughts in here:
1) Weight. I was sensitive to this issue as well - how much weight can you reasonably remove before stability is impacted? A few years back I recall reading a story of a Chris-Craft 57' Constellation that was repowered from the original 8V71Ns to modern Yanmars. It was a charter boat and apparently the weight reduction was significant enough that it failed the stability tests. When I repowered a couple of guys recommended I replace the VT8-370Ms with Caterpillar 3208TAs as they were the closest match in terms of hp, size and weight that I would be able to reasonably find. The Cummins with gears were 2775lbs each and the Caterpillar 3208TAs with the ZF gears I installed are around 2080lbs, so I still lost close to 1400lbs total. The nice thing about this weight loss is that it offset the weight of large waterlift mufflers, added batteries, inverter, larger waste tanks, toolboxes and a pile of other stuff that I added to the pump room and engine room.
2) Calculating Performance. I used the calculator on BoatDiesel. Use your existing sea trial numbers to figure out the hull constants and then start plugging in new engine scenarios. It's reasonably accurate based on my experience - I did not need to repitch the props I selected. If you are getting a yard to repower they should work with the engine distributor to calculate gearing, shaft diameter and prop dimensions and give you projected cruise and WOT speeds.

How did you get the old engines out and the new ones in? I'm thinking of repowering but dread cutting the cabin top open for access.

 

[Timeless72]

Thanks for the info website suggestion! I'll pick up a membership.

 

[Cricket]

If you're worried about CG just replace with higher horsepower engines that are the same weight, etc.
Or, you could add a Seakeeper. Haha. Btw, I can't believe how many gyros are getting installed these days.
Most likely your boat is a lot heavier now than the day it left the factory because it probably has more stuff now than then.

 

[SKYCHENEY]

If you're worried about CG just replace with higher horsepower engines that are the same weight, etc.
Or, you could add a Seakeeper. Haha. Btw, I can't believe how many gyros are getting installed these days.
Most likely your boat is a lot heavier now than the day it left the factory because it probably has more stuff now than then.

After repower, I figure I lost zero weight. The new bigger shafts and struts added just as much weight as I lost in the engine/trans package. But, that's with going up in horsepower.

 

[warrenw]

How did you get the old engines out and the new ones in? I'm thinking of repowering but dread cutting the cabin top open for access.

Eric,

The cabin top was cut open. Thankfully Chris-Craft framed the cabin top to accommodate this - pull headliner, drill up into the fiberglass skin from corners of fidley hatch framing, cut straight between the holes, pull fasteners out of the framing and the fidley hatch is free. Reinstall with fasteners then refiberglass. I'm not sure if Hatteras had a similar setup in the framing.

 

[Pascal]

I lost a lot of weight going from 8V71s to 6CTAs, it s not just engines but also the gears as the Alisons weight like 350 lbs more than the ZFs. Each. The boat does feel a little more tender especially with low tanks but I would not say unstable. I still have to do the hydraulics for the new Trac stabs and I m sure that’s going to help.

 

[Boatsb]

Stabilizers do not change the cob/cog numbers. It's dangerous to count on Stabilizers for seaworthyness. If a vessel is not seaworth with changes in weight and bouncy it needs to I be corrected. Ballast or other options should be considered