Thread: Bolger micro trawler modification

HAve you brought the Plans yet??

Any plans can be modified, with caution, and fore thought, As I am not familiar with the design, I have just had a look at the design, yes you could delete the rear cabin, , but the design does not lend it self to an inboard motor set up.

Jeff
vk4

Originally Posted by vk4

HAve you brought the Plans yet??

Any plans can be modified, with caution, and fore thought, As I am not familiar with the design, I have just had a look at the design, yes you could delete the rear cabin, , but the design does not lend it self to an inboard motor set up.

Jeff
vk4

I will bring them next week.As you said,modification can be done with caution.Offcourse I will check the weight of inboard engine and
outboard.If there is no such a big difference,cancealed sleeping area section weight will compensate the extra inboard engine weight.

Yes it may compensate for the weight difference , but it will change the balance of the boat , which was designed for an outboard , you will be moving weight forward. and this may make the hull light at the stern.

I would be looking at this aspect , as you could end up bow heavy.

jeff
vk4

One of the first things you do when designing a boat is a weight study, to figure out where things need to live, to get the boat to float where you want, without a bow up or down trim. For a novice, this can be a pain in the butt, especially if you don't have a few key "centers" to work with. Bolger usually marked the CB on his plans, so scale this as a distance from the bow perpendicular. It needs to stay there, so any changes you make will permit the boat to trim properly.

Without going into moment to trim and weight to immersion calculations (actually fairly easy), the easiest way to approach your changes, are to just go ahead and plan the inboard install. Now, you will be moving weight off the stern and more importantly the cantilever this causes. With the inboard, shaft, strut, maybe a skeg and rudder installed, launch the boat. It would be wise to make evenly spaced marks on the bow and stern, maybe with some tape marked every inch or centimeter. This will tell you how far off your trim is, compared to what it's supposed to be. It's very likely you'll be down at the bow a bit, maybe a few inches. No big deal, you'll counter balance, by moving something aft, like a fuel tank or batteries (or both). The nice thing about this is, it usually doesn't take much, because of the cantilever thing.

Boats balance on this thing called the CB (Center of Buoyancy). Okay, where is the darned CB, well, on your boat it'll be about 55% aft of the front of the LWL. Divide the length of the LWL up, then figure out how far back 55% of this figure is and that's about where you're CB will be. It's handy to know this location so you an roughly figure how much weight you need to make the boat balance. All boats teeter totter over the CB, just like a seasaw.

For example, lets say you're 3" (76 mm) down at the bow on pre-launch/test day and you place 100 pounds (45 k) of sand bags 5' (1.5 m) aft of the CB. This causes 500 pounds (227 k) of leverage over the boat, making her to trim down at the stern. Well, this may be the wrong location for you or maybe just too much weight, you want to hide in this spot. This is where the cantilever thing comes in, because to get the same leverage you can use just 50 pounds (23 k), but placed 10' aft of the CB.

The average starting battery is about 35 - 45 pounds (16 - 20 k) so this is a help. A full 10 gallon fuel tank is about 62 pounds (28 k). Both of these are likely close enough to the weight of the outboard and tank to help offset the bow down trim issue.

I guess what I'm saying in a wordy, round about way is, don't worry so much about it, until you splash the boat. A straight shaft setup, will have a rudder and quadrant that the outboard doesn't, so you've put some weight back and the shaft and strut will also add some weight, to where the outboard used to live, not to mention the wheel (prop) and it's likely the engine itself will also be aft of the CB, further helping keep the stern down, so you're not going to need as much weight or rearranged gear as you think.

A pre-launch/test day is always a good thing. Only invite well trusted (that can keep their mouths shut) and willing to work folks, as it's not launch day, just some testing and checking stuff out day, (like big leaks because you forgot to close or seal something). These are not things you want the better half to see on launch day, with the national anthem playing and all your best friends around, (trust me on this). It'll give you a chance to sort things out, make some adjustments and most importantly prevent major embarrassment, when the real launch day comes around. I've seen very few back yard builds go off without a hitch on launch day, so expect something (probably more then one) thing to go wrong. You can nip most of these things off, with a pre-launch/test day, so you look like a hero on the real launch day.

Another option might be a V-drive to keep the engine weight further back.
MicroTrawler's stepped hull might also lend itself to a jet drive, which could also bring the engine aft a bit.
Or a stern-drive to combine the best & worst of outboard & inboard drives.

Originally Posted by PAR

One of the first things you do when designing a boat is a weight study, to figure out where things need to live, to get the boat to float where you want, without a bow up or down trim. For a novice, this can be a pain in the butt, especially if you don't have a few key "centers" to work with. Bolger usually marked the CB on his plans, so scale this as a distance from the bow perpendicular. It needs to stay there, so any changes you make will permit the boat to trim properly.

Without going into moment to trim and weight to immersion calculations (actually fairly easy), the easiest way to approach your changes, are to just go ahead and plan the inboard install. Now, you will be moving weight off the stern and more importantly the cantilever this causes. With the inboard, shaft, strut, maybe a skeg and rudder installed, launch the boat. It would be wise to make evenly spaced marks on the bow and stern, maybe with some tape marked every inch or centimeter. This will tell you how far off your trim is, compared to what it's supposed to be. It's very likely you'll be down at the bow a bit, maybe a few inches. No big deal, you'll counter balance, by moving something aft, like a fuel tank or batteries (or both). The nice thing about this is, it usually doesn't take much, because of the cantilever thing.

Boats balance on this thing called the CB (Center of Buoyancy). Okay, where is the darned CB, well, on your boat it'll be about 55% aft of the front of the LWL. Divide the length of the LWL up, then figure out how far back 55% of this figure is and that's about where you're CB will be. It's handy to know this location so you an roughly figure how much weight you need to make the boat balance. All boats teeter totter over the CB, just like a seasaw.

For example, lets say you're 3" (76 mm) down at the bow on pre-launch/test day and you place 100 pounds (45 k) of sand bags 5' (1.5 m) aft of the CB. This causes 500 pounds (227 k) of leverage over the boat, making her to trim down at the stern. Well, this may be the wrong location for you or maybe just too much weight, you want to hide in this spot. This is where the cantilever thing comes in, because to get the same leverage you can use just 50 pounds (23 k), but placed 10' aft of the CB.

The average starting battery is about 35 - 45 pounds (16 - 20 k) so this is a help. A full 10 gallon fuel tank is about 62 pounds (28 k). Both of these are likely close enough to the weight of the outboard and tank to help offset the bow down trim issue.

I guess what I'm saying in a wordy, round about way is, don't worry so much about it, until you splash the boat. A straight shaft setup, will have a rudder and quadrant that the outboard doesn't, so you've put some weight back and the shaft and strut will also add some weight, to where the outboard used to live, not to mention the wheel (prop) and it's likely the engine itself will also be aft of the CB, further helping keep the stern down, so you're not going to need as much weight or rearranged gear as you think.

A pre-launch/test day is always a good thing. Only invite well trusted (that can keep their mouths shut) and willing to work folks, as it's not launch day, just some testing and checking stuff out day, (like big leaks because you forgot to close or seal something). These are not things you want the better half to see on launch day, with the national anthem playing and all your best friends around, (trust me on this). It'll give you a chance to sort things out, make some adjustments and most importantly prevent major embarrassment, when the real launch day comes around. I've seen very few back yard builds go off without a hitch on launch day, so expect something (probably more then one) thing to go wrong. You can nip most of these things off, with a pre-launch/test day, so you look like a hero on the real launch day.

Sir,
Appreciate your comments and I will study above.

Originally Posted by b.o.a.t.

Another option might be a V-drive to keep the engine weight further back.
MicroTrawler's stepped hull might also lend itself to a jet drive, which could also bring the engine aft a bit.
Or a stern-drive to combine the best & worst of outboard & inboard drives.

Would to inform you that,with V drive shape,I think,I can not reach to high speed with small engines.For jet
drive,yes it is one option and it is necessary to study.Stern drive is also another option.

On that boat, I think a jet or outdrive will eat up way too much cockpit space to be practical. An inboard, especially if small, with a jack shaft to an outdrive leg, is an option, though I don't think the hull has sufficient depth to permit the shaft to live under the cockpit sole. A jet will truly not like puttering around at low (relatively) speeds and would be my last choice for a hull shaped like this.

A straight shaft setup (as envisioned) could live near midship, with everything else below the sole, though I question the value of this, it'll leave the least foot print within the accommodations aboard.

As to a V drive, well this just moves the engine aft a bit, again taking up valuable cockpit space. A V drive's (or any other drive arrangement) ability to get some preformance from this hull form, is dependent on several factors and the boat certainly could preform just as well with it, as any other setup, assuming she's sized, geared and propped appropriately.

Originally Posted by PAR

On that boat, I think a jet or outdrive will eat up way too much cockpit space to be practical. An inboard, especially if small, with a jack shaft to an outdrive leg, is an option, though I don't think the hull has sufficient depth to permit the shaft to live under the cockpit sole. A jet will truly not like puttering around at low (relatively) speeds and would be my last choice for a hull shaped like this.

A straight shaft setup (as envisioned) could live near midship, with everything else below the sole, though I question the value of this, it'll leave the least foot print within the accommodations aboard.

As to a V drive, well this just moves the engine aft a bit, again taking up valuable cockpit space. A V drive's (or any other drive arrangement) ability to get some preformance from this hull form, is dependent on several factors and the boat certainly could preform just as well with it, as any other setup, assuming she's sized, geared and propped appropriately.

Sir,
In addition to you above comments I just saw one DIY diesel outboard(look like outboard) for boats like MT and attached below.Pls make your comments.It seems reasonable and avoid any main body modification.


Boat motor | Facebook

The Bolger Micro Trawler is a small boat, just 14' 6" (4.42 m), which is why an outboard is highly recommended. More cockpit volume could be had with a bracket.

The maximum HP rate for this boat is 50, which is about 240.00 lbs. (109 kg) on average. Add a start battery and small fuel tank and you have 340 pounds (155 kg) on the stern. I'm not sure what's available in diesel, but it's going to be heavier for sure. A typical 50 HP diesel will be well over 500 pounds, plus tanks and battery. An air cooled diesel will shave about 100 pounds off this weight. So, mounting a diesel as an inboard is logical, if only to compensate for the weight.

As to a DIY diesel outboard, well I'm not going to comment on a home brew drive assembly, but to be frank, I suspect it's not as good in many regards, as a manufactured gas outboard, in price, convolution and weight.

At the other end of the spectrum is the 25 HP minimum engine requirement. A typical 25 HP diesel will be in the 225 pound range. A gas outboard will be about 170 pounds.

In the end, I think a happy medium could be had with a 35 HP inboard, which still keeps the weight down and has it placed in a favorable location (midship).

To be honest, I don't think a boat this size is well suited to an inboard, but it's certainly possible and once was quite common.

Originally Posted by PAR

The Bolger Micro Trawler is a small boat, just 14' 6" (4.42 m), which is why an outboard is highly recommended. More cockpit volume could be had with a bracket.

The maximum HP rate for this boat is 50, which is about 240.00 lbs. (109 kg) on average. Add a start battery and small fuel tank and you have 340 pounds (155 kg) on the stern. I'm not sure what's available in diesel, but it's going to be heavier for sure. A typical 50 HP diesel will be well over 500 pounds, plus tanks and battery. An air cooled diesel will shave about 100 pounds off this weight. So, mounting a diesel as an inboard is logical, if only to compensate for the weight.

As to a DIY diesel outboard, well I'm not going to comment on a home brew drive assembly, but to be frank, I suspect it's not as good in many regards, as a manufactured gas outboard, in price, convolution and weight.

At the other end of the spectrum is the 25 HP minimum engine requirement. A typical 25 HP diesel will be in the 225 pound range. A gas outboard will be about 170 pounds.

In the end, I think a happy medium could be had with a 35 HP inboard, which still keeps the weight down and has it placed in a favorable location (midship).

To be honest, I don't think a boat this size is well suited to an inboard, but it's certainly possible and once was quite common.

I think 25HP air cooled diesel Engine will satisfy my requirement.The weight is approx.65-70 kgs maybe 75 kgs.I think one outboard with the same power is around 55-60 kgs or 70 kgs.These are all without fuel tank.One rear cabin weight
is appr.25 kgs which will not be built and compensate the access volume,I think.I will also enlarge the lenght of MT to 4.75 or 4.90 mts.50 cms more.If I am not wrong,it will be advantage also.

25 HP is the minimum rating for this boat and most everyone I've heard that has built one, wishes they'd put on a bigger engine, which is why I recommend the 35 HP. A 30 HP would be also a good, economical choice, offering some "punch" for chop, contrary currents and windage.

What engine are you looking at, as I'm finding 85+ kg, typical weight for air cooled, 25 HP diesels, before accessories. I have seen a Farymann that was light, but this was "naked".

Originally Posted by PAR

25 HP is the minimum rating for this boat and most everyone I've heard that has built one, wishes they'd put on a bigger engine, which is why I recommend the 35 HP. A 30 HP would be also a good, economical choice, offering some "punch" for chop, contrary currents and windage.

What engine are you looking at, as I'm finding 85+ kg, typical weight for air cooled, 25 HP diesels, before accessories. I have seen a Farymann that was light, but this was "naked".

Would like to inform you that one of my friend bought chineese originated air cooled diesel and one mechanic installed
the shaft.Offcourse block of the engine is made of Aliminium.I mean,light engine and not more than 80 kgs.They are very very cheap and no problem.Seller said,change the oil in time and go.If something happens,throw it and buy a new one.
Offcourse the above mentioned German engine is famous,but chinees are very cheap in comperision with German Engines.
Yes I am agree with your power information,A little bit bigger HP is better for unexpected current and windage.
I again thank you for your valuable information and cooperation.